On the subject of GC content. I am under the impression that structural RNAs
such as 16S rRNAs NEED a high GC content, due to the stronger bonds offered
by these bases, in order to hold together at high temperatures. I have been
told that Sulfolobus solfataricus (a hyperthermophile) has a genomic GC content
of about 36 percent, whereas it's 16S gene is about 65 percent GC. The
genome is held together by some *proteins* (don't ask me), so the mutational
selection acting on the structural RNAs is not felt by the rest of the genome.
I don't have any references, but maybe my witterings might stir up some other
the natural history museum,
london SW7 5BD.