A Stereospecific Pathway Diverts β-Oxidation Intermediates to the Biosynthesis of Rhamnolipid Biosurfactants

Rhamnolipids are multipurpose surface-active molecules produced by the bacterium Pseudomonas aeruginosa from L-rhamnose and R-3-hydroxyalkanoate (C-10 +/- 2) Precursors. R-3-hydroxyalkanoate precursor is believed to be synthesized de novo. We demonstrate, however, that beta-oxidation is the predominant source of this precursor. Inhibition of beta-oxidation sharply decreases rhamnolipids production, even when using a nonfatty acid carbon source (glycerol). Isotope tracing shows that beta-oxidation intermediates are direct precursors of rhamnolipids. A mutant-based survey revealed an operon coding for enoyl-CoA hydratases/isomerases (ECH/l), named RhlYZ, implicated in rhamnolipids production via an axial role in 3-hydroxyalkanoate synthesis. In vitro, RhlZ is an R-ECH/l transforming 2-decenoyl-CoA, a beta-oxidation intermediate, into R-3-hydroxydecanoyl-CoA, the potential rhamnolipids precursor. Interestingly, polyhydroxyalkanoates share with rhamnolipids the RhlYZ-generated R-3-hydroxyalkanoates pool, as demonstrated by the decrease of polyhydroxyalkanoates upon mutation of rhlYZ and the increase of rhannnolipids in a polyhydroxyalkanoates-defective mutant.