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Summary

N2 fixation is an O2-sensitive process and some filamentous diazotrophic cyanobacteria that grow performing oxygenic photosynthesis confine their N2 fixation machinery to heterocysts, specialized cells that maintain a reducing environment adequate for N2 fixation. Respiration is thought to contribute to the diazotrophic metabolism of heterocysts and the genome of the heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 bears three gene clusters putatively encoding cytochrome c oxidases. Transcript analysis of these cox gene clusters through RNA/DNA hybridization identified two cox operons, cox2 and cox3, that are induced after nitrogen step-down in an NtcA- and HetR-dependent manner and appear to be expressed specifically in heterocysts. In contrast, cox1 was expressed only in vegetative cells. Expression of cox2 and cox3 occurred at an intermediate stage (about 9 h) during the process of heterocyst development following nitrogen step-down. Inactivation of genes in the two inducible cox operons, but not separately in either of them, strongly reduced nitrogenase activity and prevented diazotrophic growth in aerobic conditions. These results show that the nitrogen-regulated cytochrome c oxidase-type respiratory terminal oxidases Cox2 and Cox3 are essential for heterocyst function in Anabaena sp. PCC 7120.