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Background   Pseudomonas aeruginosa  produces rhamnolipid biosurfactants involved in numerous phenomena including virulence. The transcriptional study of the  rhlAB  operon encoding two key enzymes for rhamnolipid synthesis led to the discovery of the quorum sensing system RhlRI. The latter positively controls the transcription of  rhlAB , as well as of  rhlC , which is required for di-rhamnolipid synthesis. The  rhlG  gene encodes an NADPH-dependent β-ketoacyl reductase. Although it was reported to be required for the biosynthesis of the fatty acid part of rhamnolipids, its function in rhamnolipid synthesis was later questioned. The  rhlG  transcription and its role in rhamnolipid production were investigated here.    Results  Using 5′-RACE PCR, a  luxCDABE -based transcriptional fusion, and quantitative reverse transcription-PCR, we confirmed two previously identified σ 70 - and σ 54 -dependent promoters and we identified a third promoter recognized by the extra-cytoplasmic function sigma factor AlgU.  rhlG  was inversely regulated compared to  rhlAB  and  rhlC : the  rhlG  transcription was down-regulated in response to  N -butyryl- l -homoserine lactone, the communication molecule of the RhlRI system, and was induced by hyperosmotic stress in an AlgU-dependent manner. Consistently with this transcriptional pattern, the single or double deletions of  rhlG  and PA3388, which forms an operon with  rhlG , did not dramatically impair rhamnolipid synthesis.    Conclusion  This first detailed study of  rhlG  transcription reveals a complex regulation involving three sigma factors and  N -butyryl- l -homoserine lactone. We furthermore present evidences that RhlG does not play a key role in rhamnolipid synthesis.

The Pseudomonas aeruginosa rhlG and rhlAB genes are inversely regulated and RhlG is not required for rhamnolipid synthesis