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Whole
Genome Transplantation Achieved in Mycobacterium
June 28, 2007
Researchers at the J. Craig
Venter Institute (JCVI) today announced the results of work on genome
transplantation methods allowing them to transform one type of bacteria
into another type dictated by the transplanted chromosome. The work,
published online in the journal Science, by JCVI’s Carole Lartigue, Ph.D.
and colleagues, outlines the methods and techniques used to change one
bacterial species, Mycoplasma capricolum into another, Mycoplasma mycoides
Large Colony (LC), by replacing one organism’s genome with the other one’s
genome.
“The successful completion of this
research is important because it is one of the key proof of principles in
synthetic genomics that will allow us to realize the ultimate goal of
creating a synthetic organism,” said J. Craig
Venter, Ph.D.,
president and chairman, JCVI.
“We are committed to this research as
we believe that synthetic genomics holds great promise in helping to solve
issues like climate change and in developing new sources of energy.”
Methods and techniques
used
The JCVI team devised several key steps to enable the genome
transplantation. First, an antibiotic selectable marker gene was added to
the M. mycoides LC chromosome to allow for selection of living cells
containing the transplanted chromosome. Then the team purified the DNA or chromosome
from M. mycoides LC so that it was free from proteins (called naked DNA).
This M. mycoides LC chromosome was then transplanted into the M. capricolum
cells. After several rounds of cell division, the recipient M. capricolum
chromosome disappeared having been replaced by the donor M. mycoides LC
chromosome, and the M. capricolum cells took on all the phenotypic
characteristics of M. mycoides LC cells. As a test of the success of the
genome transplantation, the team used two methods — 2D gel electrophoresis
and protein sequencing, to prove that all the expressed proteins were now
the ones coded for by the M. mycoides LC chromosome. Two sets of antibodies
that bound specifically to cell surface proteins from each cell were
reacted with transplant cells, to demonstrate that the membrane proteins
switch to those dictated by the transplanted chromosome not the recipient
cell chromosome. The new, transformed organisms show up as bright blue
colonies in images of blots probed with M. mycoides LC specific antibody.
The group chose to work with these species of mycoplasmas for several
reasons — the small genomes of these organisms which make them easier to
work with, their lack of cell walls, and the team’s experience and
expertise with mycoplasmas. The mycoplasmas used in the transplantation
experiment are also relatively fast growing, allowing the team to ascertain
success of the transplantation sooner than with other species of
mycoplasmas.
According to Dr. Lartigue, “While we are excited by the results of our research, we are
continuing to perfect and refine our techniques and methods as we move to
the next phases and prepare to develop a fully synthetic chromosome.”
Genome transplantation is an essential enabling step in the field of
synthetic genomics as it is a key mechanism by which chemically synthesized
chromosomes can be activated into viable living cells. The ability to
transfer the naked DNA isolated from one species into a second microbial
species paves the way for next experiments to transplant a fully synthetic
bacterial chromosome into a living organism and if successful, “boot up” the new entity. There are
many important applications of synthetic genomics research
including development of new energy sources and as means to produce
pharmaceuticals, chemicals or textiles.
Background and Ethical Considerations
The
work described by Lartigue et al. has its genesis in research begun by Dr.
Venter and colleagues in the mid-1990’s after sequencing Mycoplasma
genitalium and beginning work on the minimal genome project. This area of
research, trying to understand the minimal genetic components necessary to
sustain life, underwent significant ethical review by a panel of experts at
the University of Pennsylvania (Cho et al, Science December 1999:Vol. 286.
no. 5447, pp. 2087 – 2090). The bioethical group's independent
deliberations, published at the same time as the scientific minimal genome
research, resulted in a unanimous decision that there were no strong
ethical reasons why the work should not continue as long as the scientists
involved continued to engage public discussion. In 2003 Drs. Venter,
Smith and Hutchison made the first significant strides in the development
of a synthetic genome by their work in assembling the 5,386 base pair
bacteriophage φX174 (phi X).
They did so using short, single strands of synthetically produced,
commercially available DNA (known as oligonucleotides) and using an
adaptation of polymerase chain reaction (PCR), known as polymerase cycle
assembly (PCA), to build the phi X genome. The team produced the synthetic
phi X in just 14 days. Dr. Venter and the team at JCVI
continue to be concerned with the societal implications of their work and
the field of synthetic genomics generally. As such, the Institute’s policy
team, along with the Center for Strategic & International Studies
(CSIS), and the Massachusetts Institute of Technology (MIT), were funded by
a grant from
the Alfred P. Sloan Foundation for a 15-month study to explore the risks
and benefits of this emerging technology, as well as possible safeguards to
prevent abuse, including bioterrorism. After several workshops and public
sessions the group is set to publish a report in summer 2007 outlining
options for the field and its researchers.
About the J. Craig
Venter Institute
The
J. Craig
Venter Institute is a not-for-profit research institute dedicated to the
advancement of the science of genomics; the understanding of its
implications for society; and communication of those results to the
scientific community, the public, and policymakers. Founded by J. Craig
Venter, Ph.D.,
the JCVI is home to approximately 500 scientists and staff with expertise
in human and evolutionary biology, genetics, bioinformatics/informatics,
information technology, high-throughput DNA sequencing, genomic and
environmental policy research, and public education in science and science
policy. The legacy organizations of the JCVI are: The Institute for Genomic
Research (TIGR), The Center for the Advancement of Genomics (TCAG), the
Institute for Biological Energy Alternatives (IBEA), the Joint Technology
Center (JTC), and the J.
Craig Venter Science Foundation.
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