In addition to the above, it is also plausible that the selective forces known to operate on nucleotide composition of genomes (Hershberg and Petrov 2010; Hildebrand et al. 2010) are, at least in part, driven by their downstream effects on proteome composition (as determined by the genetic code).
Indeed, it appears that the strength of the evolutionary constraints operating at the nucleotide level are not equal between the isoforms, with H2A.Z-2 being significantly more constrained than H2A.Z-1 [ 24].
Since these races do not self-fertilize, this suggests that gene conversion or another mechanism might be operating to reduce the nucleotide diversity within their genomes over time.
Most mutagenesis techniques operate at the nucleotide level, and so frequently introduce single-nucleotide codon changes (e.g., GGA → cGA) but only very rarely introduce multi-nucleotide codon changes (e.g., GGA → cat).
Clonal groups should be defined using nucleotide sequences, and not amino acids, since the rearrangement process and SHM operate at the nucleotide level.
Despite differences between the forward and reverse nucleotide mutation rates operating in primate genomes, the general time reversible (hereafter GTR, [ Lanave et al. 1984]) variant of the conditional nucleotide frequency (hereafter CNF) codon substitution model successfully distinguished neutral mutation processes from selection in primates (Yap et al. 2010).
Extensive analysis has focused on ATP-driven conformational changes of Hsp90; however, little is known about how Hsp90 operates under physiological nucleotide conditions in which both ATP and ADP are present.
UDG, which operates by a nucleotide flipping mechanism, was first converted into a sequence nonspecific cytosine DNA glycosylase (CDG) by altering the base-specific hydrogen bond donor-acceptor groups in the active site.
Indeed, the constraints operating on H2A.Z-1 athehe nucleotide level during this differentiation process seem to be somewhat more relaxed compared with those operating on H2A.Z-2 and H2A.Z-e.
Again our results reinforce the finding that factors other than nucleotide composition are operating in the plastid genome to select among synonymous codons across genes, in that most points do not fall on the prediction line.
