rhlA is required for the production of a novel biosurfactant promoting swarming motility in Pseudomonas aeruginosa: 3-(3-hydroxyalkanoyloxy)alkanoic acids (HAAs), the precursors of rhamnolipids

Abstract

Pseudomonas aeruginosaproduces extracellular glycolipids composed ofl-rhamnose and 3-hydroxyalkanoic acid called rhamnolipids. Although these compounds are usually regarded as biosurfactants or haemolysins, their exact physiological function is not well understood. Rhamnolipids are synthesized by a rhamnosyltransferase, encoded by therhlABoperon, which catalyses the transfer of TDP-l-rhamnose to 3-(3-hydroxyalkanoyloxy)alkanoic acid (HAA) moieties of various lengths. RhlB is the catalytic protein of the rhamnosyltransferase.rhlAis indispensable for rhamnolipid synthesis, but its function is unknown. Using a liquid chromatography/mass spectrometry method, the production of extracellular HAAs byP. aeruginosawas detected previously and it was demonstrated that they are the actual precursors of rhamnolipid biosynthesis. In this report, evidence is presented indicating thatrhlAis required for production of HAAs and that these HAAs display potent surface-active properties.P.aeruginosacan colonize surfaces by swarming motility, a form of organized translocation requiring the production of wetting agents. UsingrhlAandrhlBmutants it was observed that swarming requires the expression of therhlAgene but does not necessitate rhamnolipid production, as HAAs act as surfactants. Finally, it was shown that the use of ammonium instead of nitrate as source of nitrogen and an excess of available iron both decreaserhlAexpression and swarming motility.