Abstract
The cpmA gene mediates an output signal in the cyanobacterial circadian system. This gene and its homologues are evolutionarily old, and occur in some non-photosynthetic bacteria and archaea as well as in cyanobacteria. The gene has two functional domains that differ drastically in their level of polymorphism: the N-terminal domain is much more variable than the PurE homologous C-terminal domain. The phylogenetic tree of the cpmA homologues features four main clades (C1-C4), two of which (C1 and C3) belong to cyanobacteria. These cyanobacterial clades match respective ones in the previously reported phylogenetic trees of the other genes involved in the circadian system. The phylogenetic analysis suggested that the C3 subfamily, which comprises the genes from the cyanobacteria with the kaiBC-based circadian system, experienced a lateral transfer, probably from evolutionarily old proteobacteria about 1,000 million years ago. The genes of this subfamily have a significantly higher nonsynonymous substitution rate than those of C1 (2.13 x 10(-10) and 1.53 x 10(-10) substitutions per nonsynonymous site per year, respectively). It appears that the functional and selective constraints of the kaiABC-based system have slowed down the rate of sequence evolution compared to the cpmA homologues of the kaiBC-based system. On the other hand, the differences in the mutation rates between the two cyanobacterial clades point to the different functional constraints of the systems with or without kaiA.