In the filamentous model cyanobacterium Anabaena sp. PCC 7120 heterocyst formation is triggered by changes in the C/N-ratio and relies on transcriptional reprogramming in cells. The transcription factor PacR is thought to serve as a global regulator of carbon assimilation under photoautotrophic conditions. In response to C_i-availability, PacR may modulate the carbon concentrating mechanism and photosynthesis, balancing reducing power generation while protecting the photosynthetic apparatus from oxidative damage. However, PacR also binds to promoters of genes associated with heterocyst formation, although the underlying mechanisms remain unclear. To explore this, we studied a response of a PacR-deletion mutant to a nitrogen source shift from ammonium to nitrate. The absence of PacR led to the heterocyst formation in nitrate containing media, as well as reduced growth and chlorophyll-content. We observed impaired nitrate uptake and disrupted ammonium assimilation via the GOGAT-cycle. This phenotype may be exacerbated by reduced PSI-yield and reduced expression of ferredoxin, which may lead to less reducing equivalents for nitrogen assimilation.

Our results provide insights into the regulation of heterocyst-formation in Anabaena, potentially advancing its use in biotechnological applications that utilize heterocyst as microoxic cell factories for N2-fixation and hydrogen production.