Journal Article
Proceedings of the National Academy of Sciences, vol. 105, iss. 16, pp. 6156-6161, 2008
Authors
Jana Stöckel, Eric A. Welsh, Michelle Liberton, Rangesh Kunnvakkam, Rajeev Aurora, Himadri B. Pakrasi
Abstract
Cyanobacteria are photosynthetic organisms and are the only prokaryotes known to have a circadian lifestyle. Unicellular diazotrophic cyanobacteria such as
Cyanothece
sp. ATCC 51142 produce oxygen and can also fix atmospheric nitrogen, a process exquisitely sensitive to oxygen. To accommodate such antagonistic processes, the intracellular environment of
Cyanothece
oscillates between aerobic and anaerobic conditions during a day–night cycle. This is accomplished by temporal separation of the two processes: photosynthesis during the day and nitrogen fixation at night. Although previous studies have examined periodic changes in transcript levels for a limited number of genes in
Cyanothece
and other unicellular diazotrophic cyanobacteria, a comprehensive study of transcriptional activity in a nitrogen-fixing cyanobacterium is necessary to understand the impact of the temporal separation of photosynthesis and nitrogen fixation on global gene regulation and cellular metabolism. We have examined the expression patterns of nearly 5,000 genes in
Cyanothece
51142 during two consecutive diurnal periods. Our analysis showed that ≈30% of these genes exhibited robust oscillating expression profiles. Interestingly, this set included genes for almost all central metabolic processes in
Cyanothece
51142. A transcriptional network of all genes with significantly oscillating transcript levels revealed that the majority of genes encoding enzymes in numerous individual biochemical pathways, such as glycolysis, oxidative pentose phosphate pathway, and glycogen metabolism, were coregulated and maximally expressed at distinct phases during the diurnal cycle. These studies provide a comprehensive picture of how a physiologically relevant diurnal light–dark cycle influences the metabolism in a photosynthetic bacterium.
