Effect of controlled expression of the hetR gene on heterocyst formation in the filamentous cyanobacterium Anabaena sp. PCC 7120

Abstract

hetR is a central regulatory gene inducing and possibly maintaining irreversible heterocyst differentiation in filamentous cyanobacteria. A plasmid was constructed which enabled IPTG‐mediated, controlled expression of hetR from a ptac promoter in Anabaena. When introduced into a heterocyst‐deficient hetR mutant, induction led to massive formation of heterocysts in a medium free of combined nitrogen. In nitrate‐containing cultures, induction elicited formation of only a few heterocysts, but led to nitrogen chlorosis in vegetative cells as evidenced from degradation of phycobiliproteins. Removal of the inducer IPTG caused chlorosis and death of the organisms in nitrate‐free medium, but no reversal of heterocyst formation. This indicates that constant synthesis of HetR is not the (sole) reason for irreversibility of heterocyst formation.