Supports Idea that Dimers influence Genetic Code

I may be repeating myself, but here goes.
This excerpt supports my idea that UV caused
dimers on pyrimidines would have been a major
impact on the genetic code.

Note this excerpt. They're talking about GC content in organisms.
Singer and Ames gave numerous examples, ranging from 29% GC in
Mycoplasma to 74% GC in Actinoplanes, and in general the higher GC
organisms were those deemed to have a high UV exposure and the lower
GC organisms were the others. "Partial" candidates, those with intermediate
UV exposure, often showed accordingly intermediate GC contents.
One of their cited examples is E. coli, which has a life cycle split into
enteric (gut-dwelling) and external periods: it has a GC content of 50%.

The functional basis given for this pattern was that
UV damage is largely thymine specific, so the bias is not so much pro-GC as
anti-T.
By far the
most abundant product of UV damage is the formation of pyrimidine dimers:

roughly 10 are formed per minute in E. coli exposed to direct overhead
sunlight
at sea level, giving credence to the existence of mechanisms which protect
the
organism from the deleterious effects of sunlight.

.... but as Singer and Ames suggested, it seems unlikely that any repair
mechanism could be sufficiently efficacious to make UV damage
evolutionarily insignificant. Also, some types of UV damage are
thought to be mostly irreparable, such as DNA interchain and DNA-protein
crosslinks. Interchain crosslinks are, and DNA-protein crosslinks may
well be a thymineâ??specific phenomenon. Anyway, it seems that there
is ample room for GC content to provide additional protection: Singer
and Ames suggested that DNA with a 75% GC content has 60% fewer
photodimerization targets than DNA with a 25% GC content, while a
simple a simple calculation based on base frequencies shows a greater
difference in target frequency: at 75% GC, the probability of any given
base being a thymine is 0.125, so the chance of finding two adjacent
thymines is 0.125´ 0.125=0.015625. At 25 GC the probability of
any given base being a thymine is 0.375, so chance of finding two
adjacent thymines is 0.375´ 0.375=0.140625. The difference in
photodimerization target frequency is almost tenâ??fold by this calculation. ..

Tom


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