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MBE Advance Access published online on March 10, 2004
Molecular Biology and Evolution, doi:10.1093/molbev/msh106
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2004; all rights reserved
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Accepted January 19, 2004
© 2004 Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2004; all rights reserved.

Original Articles

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

Volodymyr Dvornyk 1*

1 Osteoporosis Research Center and Department of Biomedical Sciences, Creighton University, 601 N. 30th St., Omaha, NE 68131

* To whom correspondence should be addressed. E-mail: dvornyk{at}creighton.edu.


  Abstract

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 component of this system is a two-component sensory transduction histidine kinase sasA. Using publicly available data of complete prokaryotic genomes, I performed structural and phylogenetic analyses of the sasA genes. The obtained 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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