Circadian Rhythms of Gene Expression in Cyanobacteria

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Abstract Johnson 9633267 Organisms at all levels of biological complexity manifest circadian (daily) rhythms which are controlled by an endogenous biochemical oscillator. Many physiological and cellular processes, including sleeping/waking, body temperature, homeostatic functions, gene expression, cell division, and enzymatic activities, are regulated by these "biological clocks". In addition, these oscillators are also the timers that measure the daylength in photoperiodic timing of reproductive processes in plants and animals. Therefore, understanding the biochemical mechanism of circadian clocks is of fundamental biological interest . The biochemical nature of these biological clocks has been elusive, but their salient properties (persistence, temperature compensation, and entrainment) are conserved in all organisms in which circadian behavior has been observed, from bacteria to mammals. This suggests that the biochemical mechanism has also been conserved--if not in exact homology, then at least in terms of the general components and composition of the clock. The approach to unveiling the mechanism of circadian oscillators described in this proposal focuses on the least-complex and most technically-approachable organism in which a biological clock has been demonstrated, the cyanobacterium, Synechococcus sp. strain PCC 7942. The technical advantages of this organism are that it has a small genome which is easily manipulated genetically, and that a bioluminescent strain has been developed in which the monitoring of circadian gene expression is the most flexible and facile of any system presently available. Screening of thousands of colonies for mutations that disrupt the clock or otherwise cause aberrant rhythmic behavior is possible within a single experiment. Therefore, this cyanobacterial system has excellent tools for detailed molecular/genetic analyses and for clock investigations. This project will use this system to address three aspects of biological rhythmicity: (1) identi fication and characterization of genes that are involved in generating circadian rhythms, (2) characterization of the mechanism by which the oscillator controls rhythmic outputs, and (3) assessment of the fitness advantage that organisms derive from their circadian oscillators. %%% Organisms at all levels of biological complexity manifest circadian (daily) rhythms which are controlled by an internal biochemical oscillator. Many physiological and cellular processes, including sleeping/waking, body temperature, homeostatic functions, gene expression, cell division, and enzymatic activities, are regulated by these "biological clocks." In addition, these clocks are also the timers that measure the daylength in photoperiodic timing of reproductive processes in plants and animals. Psychiatric and medical studies have shown that circadian rhythms are involved in some forms of depressive illness, "jet lag," drug tolerance/efficacy, sleep disorders, and other aspects of human physiology. Understanding the mechanism of these biological clocks has been elusive, but one principle which has emerged is that their major properties are conserved in all organisms in which circadian behavior has been observed, from bacteria to mammals. The approach to unveiling the mechanism of circadian oscillators described in this project focuses on using the least-complex and most technically-approachable organism in which a biological clock has been demonstrated (a cyanobacterium) to discover how the clock works at a molecular level. Information gained from this work about the mechanism of circadian clocks is of fundamental biological interest and may lead to insights which will be useful in agriculture and in the diagnosis and treatment of mental health and other human disorders. *** ??