Effects of PipX on NtcA‐dependent promoters and characterization of the cox3 promoter region in the heterocyst‐forming cyanobacterium Anabaena sp. PCC 7120

In Anabaena, the pipX gene is induced in the cells differentiating into heterocysts, being the PipX factor required for full expression of late heterocyst-specific genes. Here we show that PipX has a positive effect on in vitro binding of the transcription factor NtcA to DNA, as well as on transcript production, in different NtcA-dependent promoters. We found that the cox3 operon, encoding a heterocyst-specific terminal respiratory oxidase, is expressed from three nitrogen-regulated promoters to which NtcA binds. At the three sites, NtcA binding is potentiated by PipX. Thus, PipX has a direct effect on gene expression influencing the activity of NtcA.