The LysR-type transcriptional factor PacR controls heterocyst differentiation and C/N metabolism in the cyanobacterium Anabaena PCC 7120.

PacR (All3953) has previously been identified as a global transcriptional regulator of carbon assimilation in cyanobacteria. In the facultative diazotrophic and filamentous cyanobacterium Anabaena PCC 7120 (Anabaena), inactivation of pacR has been shown to affect cell growth under various conditions. Nitrogen fixation in Anabaena occurs in heterocysts, cells differentiated semiregularly along the filaments following deprivation of combined nitrogen such as nitrate or ammonium. Here, we created a markerless deletion mutant of pacR. In addition to its growth defects observed under different light and nitrogen conditions, the mutant could form a high frequency of heterocysts, including heterocyst doublets, even in the presence of nitrate. Inactivation of pacR led to the upregulation of ntcA, a global regulator of nitrogen metabolism and heterocyst formation, as well as downregulation of genes involved in nitrate uptake and assimilation. These changes led to N-limited cells in the presence of nitrate. PacR also regulates most of the genes encoding bicarbonate transport systems. The promoter regions of ntcA, and several other genes involved in nitrogen or carbon uptake and assimilation, as well as patS and hetN involved in heterocyst patterning can be directly recognized by PacR in vitro. These findings, along with previously reported ChIP-seq data, establish PacR as a crucial transcriptional regulator for balancing carbon and nitrogen metabolism in cyanobacteria.