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Regulation of V‐nitrogenase genes in Anabaena variabilis by RNA processing and by dual repressors

Anabaena variabilis ATCC 29413 fixes nitrogen in specialized cells called heterocysts using either a Mo‐nitrogenase or a V‐nitrogenase. V‐nitrogenase structural genes, vnfDGK, as well as vnfEN form an operon with ava4025, located upstream of vnfDG that is repressed by fixed nitrogen and by Mo. The ava4025‐vnfDGKEN operon is under the control of a Mo‐repressible promoter located nearly 600 bp upstream of ava4025. Levels of vnfDG transcript were about 500‐fold higher than ava4025, the first gene of the operon. This may be the result of RNA processing at a site 87 bp upstream of vnfDG that was initially identified as the transcription start site. A strain with a deletion in the coding region of ava4025 grew diazotrophically with Mo or with V. Two similar proteins, VnfR1 and VnfR2, whose genes are located some distance from the ava4025‐vnfDGKEN operon, each repressed transcription from the ava4025‐vnfDGKEN promoter and a mutant lacking both VnfR1 and VnfR2 made the V‐nitrogenase in the presence of Mo. Overexpression of the V‐nitrogenase in the double vnfR1 vnfR2 mutant resulted in decreased activity of the Mo‐nitrogenase. VnfR1 bound specifically, in vitro, to a region upstream of the ava4025 promoter.