Homologous regulators, CnfR1 and CnfR2, activate expression of two distinct nitrogenase gene clusters in the filamentous cyanobacterium Anabaena variabilis ATCC 29413

The cyanobacterium Anabaena variabilis has two Mo-nitrogenases that function under different environmental conditions in different cell types. The heterocyst-specific nitrogenase encoded by the large nif1 gene cluster and the similar nif2 gene cluster that functions under anaerobic conditions in vegetative cells are under the control of the promoter for the first gene of each cluster, nifB1 or nifB2 respectively. Associated with each of these clusters is a putative regulatory gene called cnfR (patB) whose product has a C-terminal HTH domain and an N-terminal ferredoxin-like domain. CnfR1 activates nifB1 expression in heterocysts, while CnfR2 activates nifB2 expression. A cnfR1 mutant was unable to make nitrogenase under aerobic conditions in heterocysts while the cnfR2 mutant was unable to make nitrogenase under anaerobic conditions. Mutations in cnfR1 and cnfR2 reduced transcripts for the nif1 and nif2 genes respectively. The closely related cyanobacterium, Anabaena sp. PCC 7120 has the nif1 system but lacks nif2. Expression of nifB2:lacZ from A. variabilis in anaerobic vegetative cells of Anabaena sp. PCC 7120 depended on the presence of cnfR2. This suggests that CnfR2 is necessary and sufficient for activation of the nifB2 promoter and that the CnfR1/CnfR2 family of proteins are the primary activators of nitrogenase gene expression in cyanobacteria.