The hetC gene is a direct target of the NtcA transcriptional regulator in cyanobacterial heterocyst development

The heterocyst is the site of nitrogen fixation in aerobically grown cultur es of some filamentous cyanobacteria. Heterocyst development in Anabaena sp . strain PCC 7120 is dependent on the global nitrogen regulator NtcA and re quires, among others, the products of the hetR and hetC genes. Expression o f hetC, tested by RNA-DNA hybridization, was impaired in an ntcA mutant. A nitrogen-regulated, NtcA-dependent putative transcription start point was l ocalized at nucleotide -571 with respect to the hetC translational start. S equences upstream from this transcription start point exhibit the structure of the canonical cyanobacterial promoter activated by NtcA, and purified N tcA protein specifically bound to a DNA fragment containing this promoter. Activation of expression of hetC during heterocyst development appears thus to be directly operated by NtcA. NtcA-mediated activation of hetR expressi on was not impaired in a hetC mutant, indicating that HetC is not an NtcA-d ependent element required for hetR induction.