Cross-Activation of Two Nitrogenase Gene Clusters by CnfR1 or CnfR2 in the Cyanobacterium Anabaena variabilis.

In Anabaena variabilis, the genes, which are activated by CnfR1, produce a Mo-nitrogenase that is expressed only in heterocysts. Similarly, the genes, which are activated by CnfR2, make a Mo-nitrogenase that is expressed only in anaerobic vegetative cells. However, CnfR1, when it was expressed in anaerobic vegetative cells under the control of the promoter or from the Co-inducible promoter, activated the expression of both and . Activation of , but not , by CnfR1 required NtcA. Thus, expression of the system requires no heterocyst-specific factor other than CnfR1. In contrast, CnfR2, when it was expressed in heterocysts under the control of the promoter or from the promoter, did not activate the expression of or . Thus, activation of the system in anaerobic vegetative cells by CnfR2 requires additional factors absent in heterocysts. CnfR2 made from the promoter activated expression in anaerobic vegetative cells grown with fixed nitrogen; however, oxygen inhibited CnfR2 activation of expression. In contrast, activation of and by CnfR1 was unaffected by oxygen. CnfR1, which does not activate the promoter in heterocysts, activated the expression of the entire gene cluster from a :::: hybrid promoter in heterocysts, producing functional Nif2 nitrogenase in heterocysts. However, activity was poor compared to the normal Nif1 nitrogenase. Expression of the cluster in anaerobic vegetative cells of sp. PCC 7120, a strain lacking the nitrogenase, resulted in expression of the genes but weak nitrogenase activity. Cyanobacterial nitrogen fixation is important in the global nitrogen cycle, in oceanic productivity, and in many plant and fungal symbioses. While the proteins that mediate nitrogen fixation have been well characterized, the regulation of this complex and expensive process is poorly understood in cyanobacteria. Using a genetic approach, we have characterized unique and overlapping functions for two homologous transcriptional activators CnfR1 and CnfR2 that activate two distinct nitrogenases in a single organism. We found that CnfR1 is promiscuous in its ability to activate both nitrogenase systems, whereas CnfR2 depends on additional cellular factors; thus, it activates only one nitrogenase system.