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MBE Advance Access originally published online on March 10, 2004
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Mol. Biol. Evol. 21(8):1468-1476. 2004
DOI: 10.1093/molbev/msh106
© 2004 by the Society for Molecular Biology and Evolution. ISSN: 0737-4038

Structure and Molecular Phylogeny of sasA Genes in Cyanobacteria: Insights into Evolution of the Prokaryotic Circadian System

Volodymyr Dvornyk*, Hong-Wen Deng* and Eviatar Nevo{dagger}

* Osteoporosis Research Center and Department of Biomedical Sciences, Creighton University
{dagger} Institute of Evolution, University of Haifa, Mount Carmel, Haifa, Israel

E-mail: dvornyk{at}creighton.edu.

Cyanobacteria are the simplest organisms known to have a circadian system. In addition to the three well-studied kai genes, kaiA, kaiB, and kaiC, an important element of this system is a two-component sensory transduction histidine kinase sasA. Using publicly available data of complete prokaryotic genomes, we performed structural and phylogenetic analyses of the sasA genes. Results show that this gene has a triple-domain structure, and the domains are under different selective constraints. The sasA gene originated in cyanobacteria probably through the fusion of the ancestral kaiB gene with a double-domain, two-component sensory transduction histidine kinase. The results of the phylogenetic analyses suggest that sasA emerged before the kaiA gene, about 3,000–2,500 MYA, and has evolved in parallel with the evolution of the kaiBC cluster. The observed concordant patterns of the sasA and kaiBC evolution suggest that these genes might compose an ancient KaiBC–SasA-based circadian system, without the kaiA gene, and that such a system still exists in some unicellular cyanobacteria.

Key Words: sasA • circadian system • prokaryotes • evolution • cyanobacteria


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