Enoyl-CoA hydratase mediates polyhydroxyalkanoate mobilization in Haloferax mediterranei

Although polyhydroxyalkanoate (PHA) accumulation and mobilization are one of the most general mechanisms for haloarchaea to adapt to the hypersaline environments with changeable carbon sources, the PHA mobilization pathways are still not clear for any haloarchaea. In this study, the functions of five putative (R)-specific enoyl-CoA hydratases (R-ECHs) in Haloferax mediterranei, named PhaJ1 to PhaJ5, respectively, were thoroughly investigated. Through gene deletion and complementation, we demonstrated that only certain of these ECHs had a slight contribution to poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) biosynthesis. But significantly, PhaJ1, the only R-ECH that is associated with PHA granules, was shown to be involved in PHA mobilization in this haloarchaeon. PhaJ1 catalyzes the dehydration of (R)-3-hydroxyacyl-CoA, the common product of PHA degradation, to enoyl-CoA, the intermediate of the beta-oxidation cycle, thus could link PHA mobilization to beta-oxidation pathway in H. mediterranei. This linkage was further indicated from the up-regulation of the key genes of beta-oxidation under the PHA mobilization condition, as well as the obvious inhibition of PHA degradation upon inhibition of the beta-oxidation pathway. Interestingly, 96% of phaJ-containing haloarchaeal species possess both phaC (encoding PHA synthase) and the full set genes of beta-oxidation, implying that the mobilization of carbon storage in PHA through the beta-oxidation cycle would be general in haloarchaea.