Isolation, characterization, and expression in Escherichia coli of the Pseudomonas aeruginosa rhlAB genes encoding a rhamnosyltransferase involved in rhamnolipid biosurfactant synthesis.

Transposon Tn5-GM-induced mutant strains of Pseudomonas aeruginosa which are unable to produce rhamnolipid biosurfactants and lack rhamnosyltransferase activity have been isolated. The DNA regions flanking the transposon were cloned and used as specific probes for the isolation of the corresponding wild-type genes from a P. aeruginosa wild-type cosmid gene library. Single cosmid clones capable of restoring rhamnolipid synthesis in the mutant strains were isolated and further subcloned and sequenced, resulting in the identification of two genes (rhlAB) which are organized as an operon upstream of the previously identified rhlR regulatory gene. The RhlA protein (32.5 kDa) harbors a putative signal sequence, suggesting that this protein is located in the periplasm, while the RhlB protein (47 kDa) contains at least two putative membrane-spanning domains. The expression of the rhlAB genes was found to be enhanced 20-fold during the stationary phase of growth under conditions of nitrogen limitation, as measured by using rhlA::lacZ fusions. Moreover, the transcriptional activation of the rhlAB genes appears to depend on a functional RhlR regulatory protein. The sequence upstream of the rhlA promoter contains two inverted repeats which define putative binding sites for the RhlR regulator. The controlled expression of the rhlAB genes in Escherichia coli led to the formation of active rhamnosyltransferase. This provides direct evidence for the fact that the rhamnosyltransferase encoding genes have been identified.