By Kevin E. Noonan --
Craig Venter's colleagues at the J. Craig Venter Institute show no signs that recent vocal criticism of their work in synthetic biology has made a dent in their determination to "create life" (see "Patenting Life (Really)"). They report in this week's Science Express (an online journal of pre-publication posted papers) that they have taken the next step, by successfully transplanting an entire bacterial chromosome into a bacterial cell (Lartigue et al., "Genome Transplantation in Bacteria: Changing One Species to Another").
This work is an extension of earlier studies defined by a core set of essential genes necessary for cellular function in Mycoplasma genitalium (at left). This work is already the subject of a U.S. and International patent applications (U.S. Publication No. 2007/0122826, published May 31, 2007, and International Publication No. WO 2007/047148, published April 26, 2007), as is (or will be) the new results reported today.
In the latest work, the group reports "transplanting" the genome of one Mycoplasma species into another, using "naked" (i.e., not complexed with protein) bacterial genomic DNA and conventional polyethylene glycol-mediated transformation techniques. The transferred genome, from M. mycoides, naturally contains a tetracycline-resistance gene, permitting the recipient cells from M. capricolum to be placed under tetracycline selection, increasing yield and detection of the transformants. Importantly, the authors report that they could not detect any of the native M. capricolum DNA in the selected recipients, and the phenotype of the resulting cells was the product of M. mycoides gene expression without any contribution from (and thus no evidence of persistence of) M. capricolum DNA.
Part of the excitement about this latest feat is the size of the DNA transferred. Heretofore the largest transferred DNA has been in bacterial artificial chromosomes (BACs), having a size of about 200 kilobasepairs. The M. mycoides genome is 1.1 megabasepairs in size, an almost six-fold increase over BACs. By using tetracycline expression, the authors believe the M. capricolum DNA is gradually lost during cell divisions subsequent to transfer, leaving only M. mycoides DNA conferring the phenotype of the transgenomic cell. This was established using sensitive polymerase chain reaction (PCR) methods for detecting small amounts of residual genome-specific DNA, and hybridization assays using genome-specific repeat (transposon-related) probes.
The authors are quick to point out the limitations of their technique and are well aware of the challenges they face in producing a synthetic, genetically-engineered microorganism. This work represents only a first step, but is an important proof-of-principle. Given Dr. Venter's track record, it is very likely that we will be talking about creation of a new bacterial species (which Dr. Venter has tentatively named Mycoplasma laboratorium) comprising a truly "synthetic" minimal chromosome in a heterologous bacterial host sometime in the near future.
For additional information on this topic, please see the J. Craig Venter Institute press release.