EVIDENCE FOR REDOX REGULATION OF THE TRANSCRIPTION FACTOR NTCA, ACTING BOTH AS AN ACTIVATOR AND A REPRESSOR, IN THE CYANOBACTERIUM ANABAENAPCC-7120

NtcA has been identified as a nitrogen-responsive regulatory protein r equired for nitrogen assimilation and heterocyst differentiation in cy anobacteria. It is proposed that NtcA functions through the formation of DNA-protein complexes with its specific target sequence within the promoter regions of the regulated genes. In vitro, NtcA of Anabaena PC C 7120 binds to upstream regions of the genes whose products are invol ved in nitrogen assimilation, but also to the upstream region of rbcLS (carbon-fixation gene), xisA (encoding a site-specific recombinase ex pressed during heterocyst differentiation) and ntcA (encoding NtcA its elf). However, the mechanism by which NtcA serves as a critical regula tor for such diverse processes is not understood. With the use of elec trophoretic mobility shift assays, NtcA from Anabaena PCC 7120 was her e shown to interact with the promoter sequence of the got gene, encodi ng glutathione reductase, thereby providing a novel example of NtcA's acting as a repressor, previously found only for the rbcLS gene. Furth ermore we demonstrate that the binding of DNA. by NtcA is regulated in vitro by a redox-dependent mechanism involving cysteine residues of t he NtcA protein. These findings suggest that NtcA is a transcriptional regulator that responds not only to the nitrogen status but also to t he cellular redox status, a function that might be particularly signif icant during heterocyst differentiation.