Complete Chemical Synthesis, Assembly,
and Cloning of a Mycoplasma
genitalium Genome
Daniel G. Gibson, Gwynedd A. Benders, Cynthia Andrews-Pfannkoch, Evgeniya A. Denisova,
Holly Baden-Tillson, Jayshree Zaveri, Timothy B. Stockwell, Anushka Brownley, David W. Thomas,
Mikkel A. Algire, Chuck Merryman, Lei Young, Vladimir N. Noskov, John I. Glass, J. Craig Venter,
Clyde A. Hutchison III, Hamilton O. Smith*
We have synthesized a 582,970–base pair Mycoplasma genitalium genome. This synthetic genome,
named M. genitalium JCVI-1.0, contains all the genes of wild-type M. genitalium G37 except
MG408, which was disrupted by an antibiotic marker to block pathogenicity and to allow for
selection. To identify the genome as synthetic, we inserted “watermarks” at intergenic sites known
to tolerate transposon insertions. Overlapping “cassettes” of 5 to 7 kilobases (kb), assembled from
chemically synthesized oligonucleotides, were joined by in vitro recombination to produce
intermediate assemblies of approximately 24 kb, 72 kb (“1/8 genome”), and 144 kb (“1/4
genome”), which were all cloned as bacterial artificial chromosomes in Escherichia coli. Most of
these intermediate clones were sequenced, and clones of all four 1/4 genomes with the correct
sequence were identified. The complete synthetic genome was assembled by transformationassociated
recombination cloning in the yeast Saccharomyces cerevisiae, then isolated and
sequenced. A clone with the correct sequence was identified. The methods described here will be
generally useful for constructing large DNA molecules from chemically synthesized pieces and also
from combinations of natural and synthetic DNA segments.
M
ycoplasma genitalium is a bacterium
with the smallest genome of any independently
replicating cell that has been
grown in pure culture (1, 2). Approximately 100
of its 485 protein-coding genes are nonessential
under optimal laboratory conditions when individually
disrupted (3, 4). However, it is not known
which of these 100 genes are simultaneously dispensable.
We proposed that one approach to this
question would be to produce reduced genomes
by chemical synthesis and introduce them into cells
to test their capacity to provide the essential genetic
functions for life (4, 5). This paper describes a
necessary step toward these goals―the complete
chemical synthesis of a mycoplasma genome.
The actual synthesis and assembly of this
genome presented a formidable technical challenge.
Although chemical synthesis of genes has
become routine, the only completely synthetic
genomes so far reported have been viral (6–8).
The largest previously published synthetic DNA
that we are aware of is a 32-kb polyketide gene
cluster (9). To accomplish assembly of the
582,970–base pair (bp) M. genitalium JCVI-1.0
genome, we needed to establish convenient and
reliable methods for the assembly and cloning of
much larger synthetic DNA molecules.
Strategy for synthesis and assembly. The native
580,076-bp M. genitalium genome sequence
(Mycoplasma genitalium G37 ATCC 33530
genomic sequence; accession no. L43967) (3)
was partitioned into 101 cassettes of approximately
5 to 7 kb in length (Fig. 1) that were
individually synthesized, verified by sequencing,
and then joined together in stages. In general,
cassette boundaries were placed between genes
so that each cassette contained one or several
complete genes. This will simplify the future
deletion or manipulation of the genes in individual
cassettes. Most cassettes overlapped their
adjacent neighbors by 80 bp; however, some
segments overlapped by as much as 360 bp.
Cassette 101 overlapped cassette 1, thus completing
the circle.
Short “watermark” sequences were inserted
in cassettes 14, 29, 39, 55 and 61. Watermarks are
inserted or substituted sequences used to identify
or encode information into DNA. This information
can be either in noncoding or coding sequences
(10–12). Most commonly, watermarking has been
used to encrypt information within coding sequences
without altering the amino acid sequences
(10, 11). We opted to insert watermark sequences at
intergenicsitesbecausesynonymouscodon changes
may have substantial biological effects. Our watermarks
are located at sites known to tolerate transposon
insertions, so we expect minimal biological
effects. They allow us to easily differentiate the
synthetic genome from the native genome (2, 13).
In addition to the watermarks, a 2514-bp insertion
in gene MG408 (msrA), which includes
an aminoglycoside resistance gene, was placed in
RESEARCH ARTICLE
The J. Craig Venter Institute, Rockville, MD 20850, USA.
*To whom correspondence should be addressed. E-mail:
hsmith@jcvi.org
Fig. 1. Linear GenomBench (Invitrogen) representation of the circular 582,970-bp
M. genitalium JCVI-1.0 genome. Features shown include locations of watermarks
and the aminoglycoside resistance marker, viable Tn4001 transposon insertions
determined in our 1999 and 2006 studies (3, 4), overlapping synthetic DNA
cassettes that comprise the whole genome sequence, 485 M. genitalium proteincoding
genes, 43 M. genitalium rRNA, tRNA, and structural RNA genes, and Bseries
assemblies (Fig. 2). The red dagger on the genome coordinates line shows
the location of the yeast/E. coli shuttle vector insertion. Table S1 lists cassette
coordinates; table S2 has FASTA files for all 101 cassettes; table S3 lists watermark
coordinates; table S4 lists the sequences of the watermarks.
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cassette 89. It has been shown that a strain with
this specific defect in this virulence factor cannot
adhere to mammalian cells, thus eliminating pathogenicity
in the best available model systems (14).
The synthetic genome with all of the above insertions
is 582,970 bp in length. Figure 1 is a map
of the M. genitalium JCVI-1.0 genome showing
various features such as genes, ribosomal and
tRNAs, transposon insertions (3, 4), watermark
locations, and cassette positions.
Synthesis of DNA the size of our cassettes has
become a commodity, so we opted to outsource
their production, principally to Blue Heron Technology,
but also to DNA2.0 and GENEART. The
main challenges in this project were the assembly
and cloning of synthetic DNA molecules larger
than those previously reported. We planned a fivestage
assembly as diagrammed in Fig. 2. In the
first stage, sets of four neighboring cassettes
were assembled by in vitro recombination and
joined to a bacterial artificial chromosome (BAC)
vector DNA to form circularized recombinant
plasmids with ~24-kb inserts. For example, cassettes
1 to 4 were joined together to form the A1-4
assembly, cassettes 5 to 8 were assembled to
form A5-8, and so forth. In the second stage, the
25 A-series assemblies were taken three at a time
to form B-series assemblies. For example, B1-12
was constructed from A1-4, A5-8, and A9-12.
This reduced the 25 A-assemblies to only 8 Bassemblies,
each about 1/8 of a genome in size
(~72 kb). In the third stage, the 1/8-genome Bassemblies
were taken two at a time to make four
C-assemblies, each approximately 1/4-genome
(~144 kb) in size. These first three stages of
assembly were done by in vitro recombination
and cloned into E. coli. We encountered difficulties
in carrying out the planned assembly and
cloning of the half and whole synthetic genomes
in E. coli. For this reason, the final assemblies
were carried out in S. cerevisiae by transformationassociated
recombination (TAR) cloning.
Assembly of synthetic cassettes by in vitro
recombination. Figure 3 illustrates the reaction
used for the first stage of assembly of the overlapping
cassettes. Recombinant plasmids bearing
the individual cassette DNA inserts were cleaved
with the appropriate type IIS restriction enzymes,
which cleave outside of their recognition site to
one side, to release the insert DNA. After phenolchloroform
extraction and ethanol precipitation,
the cassettes were used without removing vector
DNA. The essential steps of the reaction are (i)
the overlapping DNA molecules are digested
with a 3′ exonuclease to expose the overlaps, (ii)
the complementary overlaps are annealed, and
(iii) the joints are repaired. Polymerase chain
reaction (PCR) amplification was used to produce
a unique BAC vector for the cloning of each assembly,
with terminal overlaps to the ends of the
assembly. Each PCR primer includes an overlap
with one end of the BAC, a Not I restriction site,
and an overlap with one end of the cassette assembly.
Cassettes were assembled, four at a time,
in the presence of the appropriate BAC vector.
Because the M. genitalium JCVI-1.0 genome
does not contain a Not I site, all of the assemblies
can be released intact from the BAC.
For example, the assembly A66-69 was constructed
by mixing together equimolar amounts of
the four cassette DNAs and the linear PCR–
Fig. 3. Assembly of cassettes by in vitro recombination. (A) Diagram of steps in the in vitro
recombination reaction, using the assembly of cassettes 66 to 69 as an example. (B) BAC vector is
prepared for the assembly reaction by PCR amplification using primers as illustrated. The linear
amplification product, after gel purification, is included in the assembly reaction of (A), such that the
desired assembly is circular DNA containing the four cassettes and the BAC DNA as depicted in (C).
Fig. 2. A plan for the five-stage assembly of the M. genitalium chromosome. In the first stage of
assembly, four cassettes are joined to make an A-series assembly approximately 24 kb in length (assembly
37-41 contained five cassettes). In the next stage, three A-assemblies are joined together to make a total
of eight ~72-kb B-series assemblies (assembly B62-77 contained four A-series assemblies). The eighthgenome
B-assemblies are taken two at a time to make quarter-genome C-series assemblies. These
assemblies were all made by in vitro recombination (see Fig. 3) and cloned into E. coli using BAC vectors.
Half-genome and whole-genome assemblies were made by in vivo yeast recombination. Assemblies in
bold boxes were sequenced to verify their correctness. For the final molecule, the D-series half molecules
were not employed. Rather, we assembled the whole molecule from the four C-series quarter molecules.
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amplified BAC vector specific for this assembly,
BAC 66-69, as described above (Fig. 3) (13). The
3′ ends of the mixture of duplex vector and cassette
DNAs were then digested to expose the overlap
regions using T4 polymerase in the absence of
2′-deoxyribonucleoside-5′-triphosphates (dNTPs).
The T4 polymerase was inactivated by incubation
at 75°C, followed by slow cooling to anneal the
complementary overlap regions. The annealed
joints were repaired using Taq polymerase and Taq
ligase at 45°C in the presence of all four dNTPs
and nicotinamide adenine dinucleotide (NAD).
[See the supporting online material for details of
the assembly reaction (13)].
Samples of the assembly reactions were subjected
to field inversion gel electrophoresis (FIGE)
to evaluate the success of the assembly (Fig. 4)
(13). Additional samples were electroporated into
E. coli EPI300 (Epicentre) or DH10B (Invitrogen)
cells and plated on LB agar plates containing
12.5 mg/ml chloramphenicol. Colonies appeared
after 24 to 48 hours. A-series assembly reactions
generally yielded several thousand colonies. B- and
C-series assembly reactions generally yielded several
hundred colonies. Colonies were picked and
BAC DNA was prepared from cultures using
an alkaline lysis procedure. The DNA was then
cleaved with Not I and analyzed by FIGE to verify
the correct sizes of the assemblies. Typically, more
than 90% of the A-series and 50% of the B- and Cseries
clones contained a BAC with the correct insert
size. Clones with the correct size were preserved
as frozen glycerol stocks. Some of the cloned assemblies
were sequenced to ascertain the accuracy
of the synthesis as indicated by bold boxes in Fig. 2.
The 25 A-series assemblies and all the larger
assemblies were cloned in the pCC1BAC vector
from Epicentre (Fig. 3). The pCC1BAC clones
could be propagated at the single-copy level in
EPI300 cells and then induced to 10 copies per
cell according to the Epicentre protocol. Induced
100-ml cultures yielded up to 200 mg of BAC
DNA. The assembly inserts in the BACs were
immediately flanked on each side by a Not I site
such that cleavage efficiently yielded the insert
DNA with part of the Not I site attached at each
end (the M. genitalium genome has no Not I sites).
When the Not I–flanked assemblies were used in
higher assemblies, the 3′ portion of the Not I site
(2 nucleotides) was removed by the chew-back
reaction. The 5′ portion of the Not I site produced
a 6-nucleotide overhang after annealing, but the
overhang was removed during repair by the Taq
polymerase 5′ exonuclease activity (Fig. 5).
B-series assemblies were constructed from
Not I–digested A-series clones, and C-series assemblies
were constructed from Not I–digested
B-series assemblies. It was generally not necessary
to gel-purify the inserts from the cleaved
vector DNA because, without complementary
overhangs, they were inactive in subsequent reactions.
FIGE analyses of the assembly reactions
for A66-69, B50-61, and C25-49 are shown in
Fig. 4, A to C. Figure 4D shows a FIGE analysis
of the sizes of these cloned inserts.
Fig. 5. Repair of annealed junctions containing nonhomologous 3′ and 5′ Not I sequences. The 3′ GC
nucleotides are removed during the chew-back reaction. In the repair reaction the 5′-GGCCGC Not I
overhangs are removed by the 5′-exonuclease activity contained in the Taq polymerase.
Fig. 4. Gel electrophoretic analyses of selected examples of A-, B-, and C-series assembly reactions and
their cloned products. (A to C) A 10-ml sample of the chew-back assembly reactions for A66-69 (A), B50-
61 (B), and C25-49 (C) was loaded onto a 0.8% Invitrogen E-gel (A and B) or onto a 1% BioRad Ready
Agarose Mini Gel (C), then subjected to FIGE using the U-5 program (A and B) or the U-9 program (13) (C).
See (13) for FIGE parameters. (D) Sizes of the Not I–cleaved assemblies were determined by FIGE analysis
as in (C). The DNA size standards were the 1-kb extension ladder (M; Invitrogen) and the low-range PFG
marker (LR PFG; NEB). Bands were visualized with a BioRad Gel Doc (A and B) or using an Amersham
Typhoon 9410 Fluorescence Imager (C and D). Unreacted cassette, A-series, B-series, and BAC DNA,
incomplete assembly products, and full-length assembly products are indicated.
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Assembly by in vivo recombination in yeast.
We were unable to obtain half-genome clones in
E. coli by the in vitro recombination procedure
described above. We suspected that larger assemblies
were simply not stable in E. coli. We
had already experienced difficulty in maintaining
the C78-101 clone except in Stbl4 E. coli cells
(Invitrogen). Thus, we turned to S. cerevisiae as a
cloning host. Yeast will support at least 2 Mb of
DNA in a linear centromeric yeast artificial chromosome
(YAC) (15) and has been used to clone
sequences that are unstable in E. coli (16).
Linear YAC clones are usually constructed by
ligation of an insert into a restriction enzyme
cloning site (17). An improvement on this method
uses cotransformation of overlapping insert
and vector DNAs into yeast spheroplasts, where
they are joined by homologous recombination
(Fig. 6A). This produces circular clones and is
known as TAR cloning (18). ATAR clone, like a
linear YAC, contains a centromere and thus is
maintained at chromosomal copy number along
with the native yeast genome. However, unlike
linear YACs, circular TAR clones can be readily
separated from the linear yeast chromosomes.
To assemble quarter genomes into halves and
wholes in yeast, we used the pTARBAC3 vector
(19). This vector contains both YAC and BAC
sequences (Fig. 6B). The vector was prepared
using a strategy similar to the one described
above for BAC vectors, but longer, 60 bp, overlaps
were generated at the termini (20). In TAR
cloning, recombination is stimulated by a factor
of about 20 at double-stranded breaks (21). Thus,
we integrated the vector at the cleaved intergenic
BsmB I site in C50-77. This resulted in the
elimination of the four bases of the BsmB I 5′
overhang. The DNA to be transformed consisted
of six pieces (one vector, two fragments of
quarter 3, and quarters 1, 2, and 4). To obtain a
full-sized genome as an insert in pTARBAC3, a
single yeast cell must take up all six pieces and
assemble them by homologous recombination.
Transformation of the yeast cells was performed
using a published method (22). Vector
and inserts were transformed at approximately
equimolar amounts. Transformants were screened
first by PCR and then by Southern blot with
mycoplasma-specific probes (13). Positive clones
were tested for stability by Southern blotting of
subclones. Based on these assays, at least 17 out
of 94 transformants screened carried a complete
synthetic genome. One of these clones,
sMgTARBAC37, was selected for sequencing.
TAR cloning was also performed with each of
the four sets of two adjacent quarter genomes, as
well as with a mixture of C1-24, C25-49, and
C50-77. DNAs from transformants of these various
experiments were isolated and electroporated
into E. coli (23). In this way, we obtained BAC
clones of the sizes expected for D1-49, D50-101,
and assemblies 25-77 and 1-77. Of these, D1-49
was chosen for sequencing, and it was correct.
Our lack of success in obtaining these clones
directly by in vitro recombination may have been
due to inefficient circularization of large DNA
molecules or to breakage during the handling of
the DNA before transforming E. coli.
Recovery of the synthetic M. genitalium
genome from yeast and confirmation of its
sequence. A 600-kb YAC is about 5% of the total
DNA in a yeast cell. To enrich sMgTARBAC37 for
sequencing, we used a strategy of total DNA isolation
in agarose, selective restriction digestion of
yeast host chromosomes, and electrophoretic separation
of these linear fragments from the large,
relatively electrophoretically immobile circular molecules
(13). Figure 6 shows the size and purity of
the sMgTARBAC37 DNA that was used to prepare
a library for sequencing. The sMgTARBAC37
DNAwas sequenced by the random shotgun method
to ~7X coverage. The sequence exactly matched
our designed genome and can be accessed at
GenBank accession number CP000925.
Error management. Our objective was to
produce a cloned synthetic genome 582,970 bp
in length with exactly the sequence we designed.
This was not trivial, because differences (errors)
between the actual and designed sequence can
arise in several ways. An error could be present in
the sequence that was supplied to the contractors.
The contractors could produce cassettes with
errors. Errors could occur during repair of the
assembly junctions. Propagation of assemblies in
E. coli or yeast could lead to errors. In the latter
two instances, errors could occur at a late stage of
the assembly. At various points during the geFig.
6. Yeast TAR cloning of the complete synthetic genome.
(A) The vector used for TAR cloning contains both BAC (shown in
blue) and YAC (shown in red) sequences (shown to scale).
Recombination of vector with insert occurs at “hooks” (shown in
green) added to the TARBAC by PCR amplification. A yeast
replication origin (ARS) allows for propagation of clones because
no ARS-like sequences (31) exist in the M. genitalium
genome. Selection in yeast is by complementation of histidine
auxotrophy in the host strain. BAC sequences allow for potential
electroporation into E. coli of clones purified from yeast. (B)
M. genitalium JCVI-1.0 quarter genomes were purified from E. coli, Not I–digested, and mixed with a
TARBAC vector for cotransformation into S. cerevisiae, where recombination at overlaps from 60 to 264
bp combined the six fragments into a single clone. The TARBAC was inserted into the BsmB I site in C50-
77. (C) CHEF gel analysis of the complete synthetic genome clone sMgTARBAC37. Size markers are the
low-range pulsed field gel marker (NEB), the host yeast strain VL6-48N (32), undigested, and the native
M. genitalium MS5 (14) genome, which contains an insertion disrupting the MG408 gene. Purified
sMgTARBAC37 from the preparation used for sequencing is shown both undigested and Not I digested.
The Not I digest releases the 583-kb synthetic M. genitalium genome from the vector. The undigested
sample confirms the circularity of the clone, because a 592-kb circle was too large to electrophorese into the
gel. A small fraction of the clone was broken, and these linear molecules were detected by a faint signal.
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nome assembly, clones were sequenced (Fig. 2).
Most of the assemblies were exactly correct;
however, in our E. coli clones, we encountered at
least one example of each of the error types
described above. Several errors were repaired by
rebuilding assemblies, but in some cases other
methods were used.
During sequence verification of the C50-77
quarter molecule, two single-bp deletions were
detected. One was traced back to a synthesis error
in cassette 65, and a corrected version was supplied
by the contractor. An error in cassette 55
resulted from an incorrect sequence transmitted
to the contractor. This cassette was corrected by
replacing a restriction fragment containing the
error with a newly synthesized fragment. C50-77
was then reassembled and sequenced. The two
errors were corrected, but two new single-base
substitution errors appeared. Taq polymerase
misincorporation in a joint region likely caused
one of these errors. The other remains unexplained
but could have arisen during propagation
in E. coli. One final reassembly yielded the correct
quarter molecule that was used to assemble
the whole chromosome.
Concluding remarks. We designed, chemically
synthesized, and assembled the entire M. genitalium
JCVI-1.0 chromosome, which is based on M.
genitalium G37, and cloned it in yeast. This
construct is more than an order of magnitude
larger than previously reported chemically synthesized
DNA products (9). The final product is
built from ~104
synthetic oligonucleotides, each
~50 nucleotides in length, and is the largest chemically
synthesized molecule of defined structure
of which we are aware.
Very large nonsynthetic constructs have previously
been produced from bacterial genomic
DNA using in vivo methods. Itaya et al. (24)
developed a method for cloning megabase-sized
segments of DNA into the Bacillus subtilis genome
using the natural transformation system
of this bacterium. They cloned almost all of the
Synechocystis PCC6803 genome as a set of four
separate ~800- to 900-kb fragments into the
B. subtilis chromosome by a reiterated “inch
worm” process to generate a composite genome.
Using a similar approach, this group recently reported
the assembly and cloning of PCR products
into an extrachromosomal vector (25). Holt et al.
(26) have described how one might reassemble
a fragmented donor genome from Haemophilus
influenzae piecewise into E. coli using, for example,
lambda Red recombination. All these methods
used sequential stepwise addition of segments to
reconstruct a donor genome within a recipient
bacterium. The sequential nature of these constructions
makes such methods slower than the purely
hierarchical scheme that we employed (Fig. 2).
Other approaches have been proposed that could
use hierarchical assembly strategies (27).
The Itaya (24) and Holt (26) groups found
that the bacterial recipient strains were unable to
tolerate some portions of the donor genome to be
cloned, for example, ribosomal RNA (rRNA)
operons. In contrast, we found that the M.
genitalium rRNA genes could be stably cloned
in E. coli BACs. We were able to clone the entire
M. genitalium genome, and also to assemble the
four quarter-genomes in a single step, using yeast
as a recipient host. However, we do not yet know
how generally useful yeast will be as a recipient
for bacterial genome sequences.
For the assembly of our synthetic genome, we
used both in vitro and in vivo recombination
methods. The efficiency of our in vitro procedure
declined as the assemblies became larger. We
were able to obtain quarter-genome, but not halfgenome,
clones using the in vitro methods described
above. Some of the larger products in the
half-genome reactions appeared to be concatamers
that formed in preference to circles. In addition,
large BACs (>100 kb) transform E. coli less
efficiently. Sheng et al. (28) found that a 240-kb
BAC transformed less efficiently by a factor of
30 than an 80-kb BAC in the same recipient
strain of E. coli.
To complete the assembly, we turned to in
vivo yeast recombination. Previous work had established
that relatively large segments (>100 kb)
of the human genome can be cloned in a circular
yeast vector if the vector carries terminal
homologies (“hooks”) that flank the human genome
segment (18). If yeast is cotransformed
with a mixture of vector and high molecular
weight human DNA, clones containing the human
DNA segment are obtained. Recombination
is stimulated by breaks at the point of homology.
We surmised that our overlapping pieces, each
of which has terminal 80-bp homologies to adjacent
pieces, might be efficiently assembled and
then joined to overlapping vector DNA by the
transformation-associated recombination mechanism
in yeast (20). We found that two quarters
could be efficiently cloned to produce half genomes
in the yeast vector. More surprisingly, four
quarters, one of which had been cleaved at the
vector insertion point, could be recombined and
cloned to yield whole genomes. This implies that
some of the competent yeast cells are capable of
taking up as many as six separate DNA molecules
and recombining them into a circular DNA
molecule. This raises the question: How many
pieces can be assembled in yeast in a single step?
The ability to assemble many pieces of DNA in a
single reaction could be very useful for generation
of combinatorial genome libraries. In the future,
it may be advantageous to make greater use of
yeast recombination to assemble chromosomes.
We are currently using a TARBAC vector to
propagate the synthetic chromosome in yeast. We
do not know whether this vector might interfere
with the production of viable cells by transplantation
(5), nor do we know whether the genomic
location of the vector could affect viability. It
may be necessary to alter the vector sequences or
even to excise the vector before transplantation.
The methods described here have advantages
compared with those previously described for
constructing large DNA molecules, either chemically
synthesized or natural. Large in vitro DNA
assemblies (>30 kb) have used type IIS restriction
enzymes to generate unique sticky ends on
the components of the assembly, which are then
joined by ligation [for example, see (9, 29)]. As the
pieces to be assembled grow larger, it becomes
increasingly difficult to find a type IIS enzyme that
does not cleave within the piece. Our method is not
limited to type IIS enzymes. We can use enzymes
that cleave infrequently, for example type II enzymes
with eight base recognition sites (e.g., Not I;
see Figs. 3 and 5) or enzymes with even greater
specificity [e.g., homing endonucleases; see New
England Biolabs (NEB) catalog]. Instead of type
IIS sticky end ligation, our method uses in vitro
recombination of overlaps between the ends of the
fragments to be assembled. A chew-back and anneal
method (Fig. 3) similar to the first step of the
assembly reaction described here was used to simultaneously
assemble and clone up to nine small
overlapping DNA fragments (275 to 980 bp) into a
plasmid vector(30). Thesecond-steprepair reaction
included in our method (13) greatly increases the
efficiency of cloning of large assemblies (>50 kb).
Nothing in our methodology restricts its use
to chemically synthesized DNA. It should be
possible to assemble any combination of synthetic
and natural DNA segments in any desired
order by designing PCR primers to generate
appropriate overlaps between them.
In closing, we wonder whether use of the
UGA codon to code for tryptophan in mycoplasmas,
rather than for termination as in the
“universal” code, contributed to our success in
cloning the synthetic M. genitalium JCVI-1.0
genome. This may make cloning in E. coli and
other organisms less toxic because most M.
genitalium proteins will be truncated. If so, then
it should be possible to synthesize other genome
constructions using this same code. The genome
would then need to be installed, for example,
by transplantation (5), in a cytoplasm that can
properly translate the UGA to tryptophan. To
generalize on this phenomenon, it might be
possible to use other codon changes as long as
there is a receptive cytoplasm with appropriate
codon usage.
Note added in proof: While this paper was
in press, we realized that the TARBAC vector in
our sMgTARBAC37 clone interrupts the gene
for the RNA subunit of RNase P (rnpB). This
confirms our speculation that the vector might
not be at a suitable site for subsequent transplantation
experiments.
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33. We thank J. Mulligan for his interest in our work and
expediting cassette synthesis by Blue Heron Technologies,
S. Vashee and R.-Y. Chuang for many helpful discussions
about the manuscript, and J. Johnson and T. Davidsen for
assistance with GenBank submissions. Additionally, we
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yeast strains and TAR cloning expertise. The bulk of
the work was supported by Synthetic Genomics, Inc.
J.C.V. is Chief Executive Officer and Co-Chief Scientific
Officer of Synthetic Genomics, Inc., a privately held
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Supporting Online Material
www.sciencemag.org/cgi/content/full/1151721/DC1
Materials and Methods
Fig. S1
Tables S1 to S4
References
15 October 2007; accepted 11 January 2008
Published online 24 January 2008;
10.1126/science.1151721
Include this information when citing this paper.
REPORTS
Asphericity in Supernova Explosions
from Late-Time Spectroscopy
Keiichi Maeda,1,2,3
* Koji Kawabata,4
Paolo A. Mazzali,2,5,6
Masaomi Tanaka,7
Stefano Valenti,8,9
Ken'ichi Nomoto,1,6,7
Takashi Hattori,10
Jinsong Deng,11
Elena Pian,5
Stefan Taubenberger,2
Masanori Iye,12
Thomas Matheson,13
Alexei V. Filippenko,14
Kentaro Aoki,10
George Kosugi,15
Youichi Ohyama,16
Toshiyuki Sasaki,10
Tadafumi Takata17
Core-collapse supernovae (CC-SNe) are the explosions that announce the death of massive stars.
Some CC-SNe are linked to long-duration gamma-ray bursts (GRBs) and are highly aspherical.
One important question is to what extent asphericity is common to all CC-SNe. Here we
present late-time spectra for a number of CC-SNe from stripped-envelope stars and use them to
explore any asphericity generated in the inner part of the exploding star, near the site of
collapse. A range of oxygen emission-line profiles is observed, including a high incidence of
double-peaked profiles, a distinct signature of an aspherical explosion. Our results suggest
that all CC-SNe from stripped-envelope stars are aspherical explosions and that SNe accompanied
by GRBs exhibit the highest degree of asphericity.
M
assive stars (≳10 solar masses) end
their lives when the nuclear fuel in
their innermost region is consumed;
lacking sufficient internal pressure support, they
can no longer withstand the pull of gravity.
Their core then collapses to a neutron star or a
black hole. Gravitational energy from the collapse
produces an explosion that expels the
rest of the star in what is observed as a supernova
(SN).
CC-SNe are classified (1) by how much of
the stellar envelope is present at the time of the
explosion (2). Stars that retain their H envelope
produce SNe with a H-rich spectrum,
classified as type II. On the other hand, stars
that have lost all or most of the H envelope
produce CC-SNe that are known as strippedenvelope
(or stripped) SNe. These include, in a
sequence of increasing degrees of envelope
stripping, type IIb (He-rich but still showing
some H), type Ib (He-rich, no H), and type Ic
(deprived of both H and He). Some type Ic
SNe (hereafter broad-lined SNe Ic) show very
broad absorption features in optical spectra obtained
within a few weeks after the explosion;
these features are produced by material moving
at a velocity > 0.1 c (c is the speed of light),
probably as the result of an explosion characterized
by a kinetic energy (EK) larger than
the canonical value of ~1051
erg (3). The most
energetic broad-lined SNe Ic can reach EK ≳
1052
erg [hereafter hypernovae (HNe) or gammaray
burst HNe (GRB-HNe)] (4) and can be
associated with GRBs (5). Figure 1 summarizes
the relation between EK and the mass of
56
Ni that powers the optical light of stripped
CC-SNe (6).
An important unsolved question concerns
how the gravitational energy of the collapse is
turned into the outward motion of the SN explosion.
Most recently proposed scenarios involve
aspherical explosions (7–11). Therefore, mapping
the explosion geometry can be illuminating. It is
especially critical to establish whether the explosion
geometry is similar for all CC-SNe or at
least for different subclasses (GRB-HNe, broadlined
SNe Ic, normal-energy SNe, or stripped versus
type II SNe).
Some evidence for asymmetric explosions
was obtained from the polarization detected in
several SNe II (12) and SNe Ib/c (13) and in the
broad-lined SN Ic 2002ap (14–16). However,
because there are still few such detections, no
systematic study exists.
1
Institute for the Physics and Mathematics of the Universe
(IPMU), University of Tokyo, Kashiwa-no-ha 5-1-5, Kashiwa
City, Chiba 277-8582, Japan. 2
Max-Planck-Institut für Astrophysik,
Karl-Schwarzschild-Strasse 1, 85741 Garching, Germany.
3
Department of Earth Science and Astronomy, College of
Arts and Science, University of Tokyo, Tokyo 153-8902, Japan.
4
Hiroshima Astrophysical Science Center, Hiroshima University,
Hiroshima 739-8526, Japan. 5
National Institute for Astrophysics–Osservatorio
Astronomica di Trieste, Via G. B. Tiepolo
11, 34143 Trieste, Italy. 6
Research Center for the Early
Universe, School of Science, University of Tokyo, Tokyo 113-
0033, Japan. 7
Department of Astronomy, School of Science,
University of Tokyo, Tokyo 113-0033, Japan. 8
Department of
Physics, University of Ferrara, I-44100 Ferrara, Italy. 9
European
Organization for Astronomical Research in the Southern
Hemisphere, Karl-Schwarzschild-Strasse 1, 85741 Garching,
Germany. 10
Subaru Telescope, National Astronomical Observatory
of Japan (NAOJ), 650 North A'ohoku Place, Hilo, HI 96720,
USA. 11
National Astronomical Observatory, Chinese Academy of
Sciences, 20A Datun Road, Chaoyang District, Beijing 100012,
China. 12
Division of Optical and Infrared Astronomy, NAOJ,
Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan. 13
National
Optical Astronomy Observatory, Tucson, AZ 85719, USA.
14
Department of Astronomy, University of California, Berkeley,
CA94720-3411,USA.15
AtacamaLargeMillimeter/Submillimeter
Array Project, NAOJ, Mitaka, Tokyo 181-8588, Japan. 16
Depart-
mentofInfraredAstrophysics,InstituteofSpaceandAstronomical
Science, Japan Aerospace Exploration Agency (JAXA), 3-1-1
Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan. 17
Astronomy
Data Center, NAOJ, Mitaka, Tokyo 181-8588, Japan.
*To whom correspondence should be addressed. E-mail:
maeda@ea.c.u-tokyo.ac.jp
29 FEBRUARY 2008 VOL 319 SCIENCE www.sciencemag.org1220
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GenomeMycoplasma genitaliumComplete Chemical Synthesis, Assembly, and Cloning of a
Noskov, John I. Glass, J. Craig Venter, Clyde A. Hutchison III and Hamilton O. Smith
N.Zaveri, Timothy B. Stockwell, Anushka Brownley, David W. Thomas, Mikkel A. Algire, Chuck Merryman, Lei Young, Vladimir
Daniel G. Gibson, Gwynedd A. Benders, Cynthia Andrews-Pfannkoch, Evgeniya A. Denisova, Holly Baden-Tillson, Jayshree
originally published online January 24, 2008DOI: 10.1126/science.1151721
(5867), 1215-1220.319Science
ARTICLE TOOLS http://science.sciencemag.org/content/319/5867/1215
MATERIALS
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REFERENCES
http://science.sciencemag.org/content/319/5867/1215#BIBL
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