A probable transcription regulator, probably involved in carbon metabolism (PubMed:15225304). Binds and probably regulates expression of bidirectional hydrogenase (hoxEFUHY), probably activates its transcription (PubMed:16102913, PubMed:16238629). A possible regulator of redox-responsive genes. Binds 2 direct repeats of the DNA with sequence 5'-CTA-N(9)-CTA-3' in the upstream region of both its own gene (includes the transcription start site) and crhR (includes the initiation codon) in a sequence-specific manner; does not bind crhR RNA (PubMed:18555801). Binds to the crhR gene, represses its transcription (PubMed:16840531). Monomer in solution, probably binds DNA as a dimer. Localized in the innermost region of the cytoplasm, associated with DNA in an evenly distributed pattern no changes are seen when cells are grown in the dark. Expressed in light grown cells, strongly repressed post-transcriptionally by UV-C light, repressed by H(2)O(2) and SeO(3) but not by SeO(4) (PubMed:15225304). Strongly expressed in dark-grown cells, induced in light-grown cells by glucose, repressed at 20 degrees Celsius (PubMed:16840531). Not induced following UV light exposure between 150 and 600 J/m(2), suggesting it does not respond to DNA damage (PubMed:15225304). Despite changes in transcript levels, protein levels are constant under light-dark regimes, anaerobic growth and in presence of glucose (at protein level) (PubMed:21642463). The N-terminal fragment (residues 1-88) binds hox promoter DNA. 5 protein spots of the same molecular weight but different pI are seen in vivo; all of them bind DNA. Probably essential, it cannot be deleted (PubMed:15225304). Depleted strain have decreased levels of hox hydrogenase (PubMed:16102913). In depletion studies using merodiploid strains 32 genes are up-regulated while 25 are down-regulated; many of them are known or predicted to be involved in carbon metabolism; cells grow poorly in the absence of inorganic carbon (PubMed:15225304). This bacterium is considerably more resistant to UV and gamma irradiation than E.coli; the E.coli-like SOS regulon model is not an appropriate model for DNA repair in this cyanobacterium. Belongs to the peptidase S24 family. Does not encode the amino acids known to be needed in E.coli for autocatalytic cleavage, probably acts differently from E.coli LexA.