Transcriptional activator of anaerobic gene expression. Regulates the expression of the components of the hydrogen cyanide synthase (HcnABC) in a positive manner (PubMed:10570200). May also act as an iron sensor. Up-regulated by Fe(3+) levels in oxygen-limiting conditions. Nearly complete loss of expression of hcnA (PubMed:10570200). Possesses 4 cysteines which may bind a metal ion (possibly iron).