"New" DNA that is taken up by transformation is incorporated into the genome
by recombination at a homologous site. If the new DNA is a strand, this
means that an "old" strand is just replaced by new, and the total DNA stays
the same. "New" DNA that is circular (normally plasmid DNA, or possibly a
strand that has been joined at the ends by a ligase) can be inserted into
the existing DNA to make a larger genome (or whole plasmids can be
transferred to a new cell). For insertion of a plasmid into the chromosome,
there still has to be some region of homology between the new and old DNA to
allow their recombination, but this can be a very short region ~15 base
pairs. Any added genetic material will require energy to replicate,
translate, etc. Any recombined "new" gene must not replace another
necessary gene to allow the organism to survive, and must give the receptor
organism some new useful ability to give any advantage to that organism, or
to keeping that gene. Microbes of course do not "know" what they are doing
or taking in on an individual basis, the "logical" results come about
through mass action: less capable individuals die and more competitive or
resilient individuals live on and replicate.
Now Roberto Verzola's questions:
The forms of horizontal gene transfer are conjugation (by physical
attachment, mostly G-'s but also some G+'s and probably many more than we
know about), transformation (uptake of foreign, free-floating DNA), and
transduction (transfer of DNA between viral hosts). Genes that are on
plasmids are much more likely to be successfully transferred to another
microbe, regardless of the mechanism, but genes on the chromosome can be
moved into a plasmid by transposons or inaccurate recombination/excision.
While genetic transfer between microbes with plasmids is fairly common,
plants are not generally a source of DNA for bacteria because plants do not
have plasmids, do not conjugate with microbes, and do not have the same
viruses. Antibiotic resistance has spread so quickly because the genes
responsible are often in transposon regions and/or plasmids. There are now
many plasmids with multiple genes for resistance to different antibiotics,
and all these genes and resistances can be transferred at once. The
original genes, though, were still of microbial (mutation) origin. There
is also tremendous pressure against pathogens that are NOT antibiotic
resistant, so again the result is due to the transformed bacteria surviving
and replicating while the originals die, not so much that all the bacteria
suddenly acquire the resistance gene(s).
Yes, there appears to be "junk" DNA, or at least material we haven't
described a purpose for. Yes, this does argue against much cost to the
organism for excess DNA, at least if it is not translated into unneeded
proteins. Maybe this is just an argument of pro-GMO people that I
internalized? Still, we are less worried about microbes picking up any ol'
junk DNA, and more concerned it will acquire intact genes and the ability to
produce toxins. If a gene is really active, it will cost the cell something
to use it.
I don't think anyone (well, any "unbiased" scientist) would claim it could
not and never will happen, but the likelihood of gaining a new functional
gene seems much greater if the source is another bacterium or actinomycete,
not a plant. There is probably more risk of spreading Bt genes to lots of
soil and phyllosphere microbes by inoculation with intact Bt bacteria than
by introduction of the toxin gene to a plant. I am more concerned about the
movement of this gene to other related plants and the deadly effects on
nontarget insects.
Lynne Carpenter-Boggs
Soil Microbiologist
USDA-ARS
803 Iowa Ave.
Morris, MN 56267
320-589-3411 x141
FAX 320-589-3787
lcboggs@mail.mrsars.usda.gov
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