Ectopic expression of hetP can partially bypass the need for hetR in heterocyst differentiation by Anabaena sp. strain PCC 7120

Abstract

Differentiation of nitrogen-fixing cells, called heterocysts, by the cyanobacterium Anabaena sp. strain PCC 7120 requires HetR, which is considered the master regulator of heterocyst differentiation. In this study, ectopic expression of hetP from an inducible promoter was shown to partially bypass the need for hetR in heterocyst differentiation. A hetR-deletion strain with hetP expressed ectopically produced cells morphologically similar to heterocysts that produced exopolysaccharide and glycolipids specific to heterocysts, and nitrogenase activity was present under anaerobic conditions, as indicated by an acetylene reduction assay. Five potential transcription start points or transcript processing sites located at positions -727, -545, -208, -177 and -12 bp relative to the putative translational start site were identified in the promoter region of hetP. Relative levels of transcription and their location in filaments resembled that previously reported for HetR protein in various genetic backgrounds, and an inverted repeat sequence in the promoter of hetP was necessary for binding of HetR. Two of the three identified homologues of hetP found in PCC 7120 partially complemented a hetP-null mutant. Taken together, these results suggest that HetP functions directly downstream of HetR in the regulatory network responsible for heterocyst differentiation.