Biomedical Engineering Reference
In-Depth Information
The techniques used to create GEMs have been discussed by Sayler and Ripp (
2000
) and Cases
and de Lorenzo (
2005
). GEMs should be particularly useful for xenobiotics that have only
recently appeared in the environment and compounds for which no degradation pathways have
been established - such as those with multiple double bonds, aromatic structures or with
multiple halogen substitutions, like polychlorinated biphenyls (PCBs) - or for compounds
that require multiple degradation steps (Khomenkov et al.,
2008
). GEMs can be optimized to
have high degradation activity. For example, the genetic elements that control the level of gene
expression, like the transcriptional promoter and terminator sequences, can be designed to
over-express the degradation genes. A similar result may be obtained by changing the number
of copies of the gene. Monitoring the location and spread of GEMs assists with both determin-
ing the success of bioaugmentation and controlling the release of GEMs. To this end, lumines-
cent tags and other methods of tracking have been implemented (Valdman et al.,
2004
).
The proposed application of GEMs is subject to some of the same public concerns as other
genetically modified organisms (GMOs), such as the unmitigated spread of the organisms,
transfer of genetic material and disruption of the natural flora (Kappeli and Auberson,
1997
;
Davison,
2005
). There are a number of ways to control the spread of GEMs and their genetic
material, but the most common is the use of molecular methods (Davison,
2005
). The horizontal
transfer of antibiotic resistance genes can be eliminated by avoiding the use of antibacterial
resistance as a selection marker during strain construction. Another partial solution to prevent
the genes from transferring to other organisms would be to avoid the use of plasmids and
maintain the genes on the chromosome, although this is not a fail-proof solution (Gentry et al.,
2004
). One control strategy, which has been implemented with GMOs, is the use of suicide
elements to biologically contain the organisms to the site and the application, as illustrated in
Figure
1.5
(Contreras et al.,
1991
; Davison,
2005
). In this system, a control element, which could
be modulated by the user, would target a killing element that would induce cell death.
While it is unlikely that any control measure to prevent GEMs from spreading will achieve
complete control, the possible benefits of GEMs for bioremediation should be weighed against
the risks. Other than contamination of industrial systems, it is unlikely that a true health risk
would evolve from the application of GEMs for pollutant degradation (Urgun-Demirtas et al.,
2006
). A recent review examined regulation of the use of GMOs in the United States,
3-methylbenzoate
3-methylbenzoate
xyl
S
xyl
S
lac
I
asd
lac
I
asd
gef
gef
Figure 1.5. Example of a control strategy for GEMs (adapted from Davison,
2005
). When the
pollutant of interest, 3-methylbenzoate, is present, it activates xylS, which then positively activates
the transcription of the asd gene (for the essential diaminopimellic acid) and lacI gene. LacI
represses the transcription of a toxin, gef. If the substrate of interest is not present, xylS is not
activated, and the cell dies from lack of diaminopimellic acid and gef toxin production.
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