Summary: spp. are marine filamentous, non-heterocystous cyanobacteria capable of aerobic nitrogen fixation. In this study, the nitrogenase structural genes and gene of sp. IMS101 were cloned and sequenced. The sp. IMS101 and amino acid sequences showed only 79%, 66% and 68% identity, respectively, to those of sp. strain PCC 7120. A potential transcription start site for was found 212 bases upstream of the start codon. Promoter-like nucleotide sequences upstream of the transcription start site were identified that were very similar to those identified for the nitrogenase genes of spp. Sequence analysis revealed regions of DNA that may form stem-loop structures in the intercistronic regions downstream of and . RNA analysis by Northern hybridization revealed the presence of transcripts corresponding to and . Surprisingly, Northern hybridization also revealed the presence of transcripts that corresponded to and , which have not been previously reported as transcripts in contiguous genes of cyanobacteria. Transcription of the genes was not significantly repressed in the presence of nitrate at a final concentration of 20 mM or at oxygen concentrations of up to 40%, whereas ammonium and urea inhibited transcription. The transcription of the genes was not affected by darkness, which suggests that transcription of these genes in is not directly regulated by light.


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