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 N2fixation cycled in continuous light but with a lower N2 fixation activity. Glycogen degradation, respiration, and photosynthesis were also lower; nonetheless, O2evolution 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.


Copyright © American Society for Microbiology, [Journal of Bacteriology, 190, 2008, 3904-3913 and 10.1128/JB.00206-08

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