34001:

Circadian clocks in cyanobacteria: mechanism and fitness.

Authors:	Johnson, Carl, H.(A)Kondo, Takao(B)Golden, Susan, S.(C)
Affiliations:	(A): Department of Biology, Vanderbilt University (B): Division of Biol. Science, Nagoya University (C): Department of Biology, Texas A&M University
Presenter:	Johnson, Carl H., carl.h.johnson@vanderbilt.edu

Circadian (daily) rhythms are ubiquitous in eukaryotes, and are also found in eubacteria among cyanobacteria. Mutational analyses of clock function in the cyanobacterium Synechococcus sp. strain PCC 7942 have discovered a circadian clock gene cluster kaiABC. Nineteen clock mutations have been mapped to the three kai genes. Promoter activities upstream of the kaiA and kaiB genes showed circadian rhythms of expression and both kaiA and kaiBC mRNAs displayed circadian cycling. Inactivation of any single kai gene abolished these rhythms and reduced kaiBC-promoter activity. Continuous kaiC-overexpression repressed the kaiBC promoter, whereas kaiA-overexpression enhanced it. Temporal kaiC-overexpression reset the phase of the rhythms. All three Kai proteins appear to interact with each other and these interactions seem to be crucial for normal clock function. Studies of Kai protein expression are underway. Apparently a negative feedback control of kaiC-expression by KaiC generates a circadian oscillation in cyanobacteria and KaiA sustains the oscillation by enhancing kaiC-expression. Previous studies have suggested that longevity, growth, and developmental rate are improved when organisms are maintained on light/dark cycles whose period is similar to the period of the endogenous circadian clock. However, some of those studies have not been reproducible and no studies have demonstrated that reproductive fitness per se is improved by consonance between the endogenous clock and the environmental cycle. Moreover, mutations of circadian clock genes in flies and fungi do not obviously impair reproductive fitness under laboratory conditions. We addressed the adaptive significance of circadian rhythmicity by testing the relative fitness under competition between various strains of cyanobacteria expressing different circadian periods. Strains that had a circadian period similar to that of the light/dark cycle were favored under competition in a manner that indicates the action of soft selection. Arhythmic strains could not compete well against wild-type strains in light/dark cycles, but they could compete effectively in constant light. These data indicate that having a circadian clock with a period that matches that of the environmental cycle enhances fitness.