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Conservation of the cyanobacterial circadian clock : comparative studies in nostoc sp. strain PCC 9709, a cyanobacterium isolated from the lichen peltigera membranacea Lee, Amy


The Cyanobacteria are the only group of prokaryotic organisms known to possess a circadian program. Efforts to elucidate the molecular mechanism underlying the cyanobacterial circadian clock have focused almost exclusively on the kaiABC gene cluster of the unicellular model organism Synechococcus elongatus PCC 7942. The considerable morphological and habitat diversity of cyanobacteria raises obvious questions: Do all members of the Cyanobacteria utilize circadian programs? How similar might other cyanobacterial clocks be to the S. elongatus model? To investigate whether clock molecular biology is conserved, I undertook a comparative study with Nostoc strain PCC 9709 as the subject. Nostoc PCC 9709 is a heterocyst-forming filamentous cyanobacterium originally isolated as a symbiont within the lichen Peltigera membranacea. Methods: The Nostoc PCC 9709 genome was screened for clock gene homologues using PCR-based methods and Southern blot hybridization. To support genetic identification with functional data, the temporal gene expression profiles of putative kai homologues were compared against the previously determined rhythmic abundances of kaiA and kaiBC transcripts of S. elongatus with the use of Northern blot hybridization and relative RT-PCR assays. Conclusions: The prokaryotic circadian clock gene cluster kaiABC is conserved in Nostoc PCC 9709. Deduced amino acid sequence homology (46 % identity for KaiA, 81 % for KaiB, 80 % for KaiC) is consistent with conservation of function in accordance with available structure-function data for the S. elongatus model. A relative RT-PCR assay method was validated on the S. elongatus model system. Time course studies on kai gene transcription demonstrated peak-and-trough patterns in cultured Nostoc and field specimens of P. membranacea that are consistent with circadian oscillation.

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