CIRCADIAN RHYTHMS - Cyanobacteria - page 329

represented in Synechococcus sp. strain WH 8102 with the three clock oscillators or Prochlorococcus
strains having only kaiB and kaiC . It is interesting to note that the trees of the kaiBC cluster and sasA
genes from the same species have similar topologies. One of the possibilities for evolution of sasA
gene sequence is by lateral gene transfer of kaiB and its fusion with the two component histidine
kinase in other Proteobacteria to form sasA and its subsequent transfer back to cyanobacteria. This
has been ruled out as the triple-domain structure homolog of sasA gene with KaiB-like sensor
domain does not occur in other prokaryotes. As kaiB genes originated only in cyanobacteria, it is
logical to conclude that sasA gene has originated in cyanobacteria apparently through fusion of
two-component histidine kinase and ancestral kaiB gene to form the currently observed triple-
domain structure (Dvornyk et al ., 2004).
According to Dvornyk et al . (2004) the ancestral circadian system consisted of kaiB and kaiC
genes which were not in a cluster. To enhance the performance of this system sasA gene supposedly
evolved as a universal input-output regulator. Due to the fusion of kaiB and kaiC , sasA must have
become an indispensable part of kaiBC - sasA circadian system. Subsequent emergence of kaiA gene
resulted in the formation of kaiABC - sasA system. Dvornyk (2005) conducted a phylogenetic analysis
of ldpA gene from the fully sequenced genomes of prokaryotes and found that the topology of the
phylogenetic tree is consistent with the phylogenetic analyses of SasA gene (Dvornyl et al ., 2004)
and kaiBC operon (Dvornyk et al ., 2003) conducted earlier by his group. These results further lend
support to the existence of KaiABC- and KaiBC-based circadian systems in prokaryotes.
The structure, polymorphism, mutation rates and selective constraints of cpmA genes have
been compared among cyanobacteria. The cpmA gene sequences have also been found in some
other bacteria and archaea. However, no cpmA gene sequences have been found in photosynthetic
α-proteobacteria ( Rhodobacter, Rhodospirillum and Rhodopseudomonas ) or in Chlorofl exus all of which
have been reported to have kai genes and/or kaiBC operon laterally transferred from cyanobacteria.
Exceptionally, Trichodesmium erythaeum possessed two copies of cpmA gene sequences while the
rest of the cyanobacteria examined had only one cpmA gene (Dvornyk et al ., 2006). Based on the
evolutionary analyses of the key components ( kaiB , kaiC and sasA genes), it was hypothesized that
each type of cyanobacterial circadian system ( kaiABC cluster or kaiBC ) has specifi c functional and
selective constraints largely because of the emergence of kaiA gene (Dvornyk et al., 2003, 2004,
2006).
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