The HetR-binding site that activates expression of patA in vegetative cells is required for normal heterocyst patterning in Anabaena sp. PCC 7120

Upon nitrogen step-down, Anabaena sp. PCC 7120 produces semi-regularly spaced heterocysts along filaments. HetR, the master regulator of heterocyst differentiation, has been shown to up-regulate hetP and hetZ in differentiating heterocysts via specific recognition sites. HetR is also predicted to bind to the promoter of patA, a gene required for heterocyst formation at intercalary positions. Here, we show that HetR binds to the predicted site 5′ from patA. Moreover, (1) deletion of the HetR-binding site greatly diminished the expression of patA in vegetative cells as shown with gfp, and (2) complementation of a patA mutant by a replicating plasmid that bears patA is largely prevented by removal of that binding site. In contrast, HetR-binding sites suppressed the expression of alr0202 (a homolog of hetZ) specifically in heterocysts and of alr3234 (a homolog of hetP) in whole filaments. Our results indicate that HetR can regulate gene expression in different modes.