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Journal of Bacteriology, June 2008, p. 3904-3913, Vol. 190, No. 11
0021-9193/08/$08.00+0     doi:10.1128/JB.00206-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Differential Transcriptional Analysis of the Cyanobacterium Cyanothece sp. Strain ATCC 51142 during Light-Dark and Continuous-Light Growth ^,

Purdue University, Department of Biological Sciences, 201 S. University Street, West Lafayette, Indiana 47907,¹ Washington University, Department of Biology, Reebstock Hall, St. Louis, Missouri 63130²

Received 9 February 2008/ Accepted 26 March 2008

We analyzed the metabolic rhythms and differential gene expression in the unicellular, diazotrophic cyanobacterium Cyanothece sp. strain ATCC 51142 under N₂-fixing conditions after a shift from normal 12-h light-12-h dark cycles to continuous light. We found that the mRNA levels of 10% of the genes in the genome demonstrated circadian behavior during growth in free-running (continuous light) conditions. The genes for N₂ fixation displayed a strong circadian behavior, whereas photosynthesis and respiration genes were not as tightly regulated. One of our main objectives was to determine the strategies used by these cells to perform N₂ fixation under normal day-night conditions, as well as under the greater stress caused by continuous light. We determined that N₂ fixation cycled in continuous light but with a lower N₂ fixation activity. Glycogen degradation, respiration, and photosynthesis were also lower; nonetheless, O₂ evolution was about 50% of the normal peak. We also demonstrated that nifH (encoding the nitrogenase Fe protein), nifB, and nifX were strongly induced in continuous light; this is consistent with the role of these proteins during the assembly of the enzyme complex and suggested that the decreased N₂ fixation activity was due to protein-level regulation or inhibition. Many soluble electron carriers (e.g., ferredoxins), as well as redox carriers (e.g., thioredoxin and glutathione), were strongly induced during N₂ fixation in continuous light. We suggest that these carriers are required to enhance cyclic electron transport and phosphorylation for energy production and to maintain appropriate redox levels in the presence of elevated O₂, respectively.

Published ahead of print on 4 April 2008.

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