Cyanide-dependent control of terminal oxidase hybridization by MpaR.

is a common, biofilm-forming pathogen that exhibits complex pathways of redox metabolism. It produces four different types of terminal oxidases for aerobic respiration, and for one of these-the -type terminal oxidases-it has the capacity to produce at least 16 isoforms encoded by partially redundant operons. It also produces small-molecule virulence factors that interact with the respiratory chain, including the poison cyanide. Previous studies had indicated a role for cyanide in activating expression of an "orphan" terminal oxidase subunit gene called and that the product contributes to cyanide resistance, fitness in biofilms, and virulence-but the mechanisms underlying this process had not been elucidated. Here, we show that the regulatory protein MpaR, which is predicted to be a pyridoxal phosphate-binding transcription factor and is encoded just upstream of , controls expression in response to endogenous cyanide. Paradoxically, we find that cyanide production is required to support CcoN4's contribution to respiration in biofilms. We identify a palindromic motif required for cyanide- and MpaR-dependent expression of and co-expressed, adjacent loci. We also characterize the regulatory logic of this region of the chromosome. Finally, we identify residues in the putative cofactor-binding pocket of MpaR that are required for expression. Together, our findings illustrate a novel scenario in which the respiratory toxin cyanide acts as a signal to control gene expression in a bacterium that produces the compound endogenously.