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 illumina tion by temporal separation of oxygen-evolving photosynthesis and oxygen-se nsitive 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 lev el of transcripts coding for phycocyanin (cpcBA) and the chlorophyll a bind ing protein of photosystem II (psbC) and psbA accompanied the depression in photosynthetic oxygen evolution. Unlike most of the other organisms examin ed to date, in P. boryanum, psbC and psbD do not appear to be co-transcribe d. The psbC transcripts were down-regulated several fold, while the psbD tr anscript declined marginally during the nitrogen fixation phase. A decrease in dissolved oxygen and a dramatic increase in the level of nifH transcrip ts and the enzyme activity of nitrogenase were characteristic of the nitrog en fixation phase. The level of transcript for glnA, which encodes glutamin e synthetase, was not altered. Reciprocal regulation of gene expression was well orchestrated with the alternating cycles of photosynthesis and nitrog en fixation in P. boryanum.