Expression of photosynthesis genes in relation to nitrogen fixation in the diazotrophic filamentous nonheterocystous cyanobacterium Trichodesmium sp. IMS 101.

The daily cycle of nitrogenase expression in the marine filamentous nonheterocystous cyanobacterium Trichodesmium spp. is controlled by a circadian rhythm. We evaluated the rhythm of two key photosynthesis genes, psbA of photosystem II and psaA of photosystem I, in Trichodesmium sp. IMS 101 using the 3 criteria for an endogenous rhythm. The transcript abundance of psbA and psaA transcripts oscillated with a period of ca. 24 h under a 12 h light/12 h dark regime. At 24 degrees C and 28 degrees C the cyclic pattern of transcript abundance was maintained for at least 58 h under constant light conditions, whereas the periods were about 24 h at 24 degrees C, and 26-30 h at the higher temperature. The cycles of psbA and psaA gene expression were entrained using light-dark cues. Transcription of nifHDK was initiated prior to the light period, followed by psbA and finally psaA. There was a 90 degrees (6 h) phase difference between the net accumulation of nifHDK and psbA transcripts, as well as between that of psbA and psaA transcripts. Results of inhibitor experiments indicated that psbA and psaA transcription was regulated differently by initiation and degradation during the light period. Short-term changes of light conditions resulted in significant effects on psbA transcription and nitrogenase activity, but had less of an effect on psaA and nifHDK transcription.