Differential expression of photosynthesis and nitrogen fixation genes in the cyanobacterium Plectonema boryanum

The filamentous non-heterocystous cyanobacterium Plectonema boryanum fixes dinitrogen at a high rate during microaerobic growth in continuous illumination by temporal separation of oxygen-evolving photosynthesis and oxygen-sensitive dinitrogen fixation. The onset of nitrogen fixation is preceded by a depression in photosynthesis that establishes a sufficiently low level of dissolved oxygen in the growth medium. A several-fold reduction in the level of transcripts coding for phycocyanin (cpcBA) and the chlorophyll a binding protein of photosystem II (psbC) and psbA accompanied the depression in photosynthetic oxygen evolution. Unlike most of the other organisms examined to date, in P. boryanum, psbC and psbD do not appear to be co-transcribed. The psbC transcripts were down-regulated several fold, while the psbD transcript declined marginally during the nitrogen fixation phase. A decrease in dissolved oxygen and a dramatic increase in the level of nifH transcripts and the enzyme activity of nitrogenase were characteristic of the nitrogen fixation phase. The level of transcript for glnA, which encodes glutamine synthetase, was not altered. Reciprocal regulation of gene expression was well orchestrated with the alternating cycles of photosynthesis and nitrogen fixation in P. boryanum.