Nitrogen availability for nitrogen fixing cyanobacteria upon growth on dinitrogen

The filamentous cyanobacterium Nostoc PCC 7120 is able to convert dinitrogen to ammonia in the absence of combined nitrogen. The expression of 20% of coding sequences from all major metabolic categories was examined in nitrogen fixing and non-nitrogen fixing growth conditions. The expression data were correlated with the nitrogen content of gene products. When growing on dinitrogen, Nostoc PCC 7120 incorporates more nitrogen into its proteome than in growth on ammonia. Thus, paradoxically, limitation of combined nitrogen in the culture medium leads to excessive nitrogen supply from the air. Biotechnological implications are discussed.   Key words: Cyanobacteria, gene expression, nitrogen availability, nitrogen fixation