An RND transporter in the monoterpene metabolism of Castellaniella defragrans.

The betaproteobacterium Castellaniella defragrans 65Phen grows on monoterpenes at concentrations toxic to many bacteria. Tolerance mechanisms include modifications of the membrane fatty acid composition and the mineralization of monoterpenes. In this study, we characterized an efflux transporter associated to the monoterpene metabolism. The inner-membrane transporter AmeD (apolar monoterpene efflux) affiliated to the HAE3 (hydrophobe/amphiphile efflux) family within the Resistance-Nodulation-Division (RND) superfamily. RND pumps of the HAE3 family are known for transporting substrates into the periplasm. AmeD is co-expressed with the outer membrane protein AmeA and the periplasmic proteins AmeB and AmeC, suggesting an export channel into the environment similar to HAE1-type RND exporters. Proteins AmeABCD are encoded within a genetic island involved in the metabolism of acyclic and cyclic monoterpenes. The deletion of ameABCD translated into a decrease in tolerance to monoterpenes in liquid cultures. The addition of acetate as cosubstrate in limonene-containing cultures partially alleviated monoterpene toxicity in the deletion mutant. Accumulation of Nile Red in cells of C. defragrans required dissipation of the proton motive force with carbonyl cyanide m-chlorophenylhydrazone (CCCP). Cells lacking AmeABCD accumulated more Nile Red, suggesting an export function of the proteins. Our observations suggest that the tetrapartite RND transporter AmeABCD acts as an exporter during monoterpene detoxification in C. defragrans.