Mechanism, regulation, and ecological role of bacterial cyanide biosynthesis

A few bacterial species are known to produce and excrete hydrogen cyanide ( HCN), a potent inhibitor of cytochrome c oxidase and several other metalloe nzymes. In the producer strains, HCN does not appear to have a role in prim ary metabolism and is generally considered a secondary metabolite. HCN synt hase of proteobacteria (especially fluorescent pseudomonads) is a membrane- bound flavoenzyme that oxidizes glycine, producing HCN and CO2. The hcnABC structural genes of Pseudomonas fluorescens and P. aeruginosa have sequence similarities with genes encoding various amino acid dehydrogenases/oxidase s, in particular with nopaline oxidase of Agrobacterium tumefaciens. Induct ion of the hcn genes of P. fluorescens by oxygen limitation requires the FN R-like transcriptional regulator ANR, an ANR recognition sequence in the -4 0 region of the hcn promoter, and nonlimiting amounts of iron. In addition, expression of the hcn genes depends on a regulatory cascade initiated by t he GacS/GacA (global control) two-component system. This regulation, which is typical of secondary metabolism, manifests itself during the transition from exponential to stationary growth phase. Cyanide produced by P. fluores cens strain CHA0 has an ecological role in that this metabolite accounts fo r part of the biocontrol capacity of strain CHA0, which suppresses fungal d iseases on plant roots. Cyanide can also be a ligand of hydrogenases in som e anaerobic bacteria that have not been described as cyanogenic. However, i n this case, as well as in other situations, the physiological function of cyanide is unknown.