Quorum-sensing-dependent regulation of biosynthesis of the polyketide antiobiotic mupirocin in Pseudomonas fluorescens NCIMB 10586

Mupirocin (pseudomonic acid) is a polyketide antibiotic, targeting isoleucyl-tRNA synthase, and produced by Pseudomonas fluorescens NCIMB 10586. It is used clinically as a topical treatment for staphylococcal infections, particularly in contexts where there is a problem with methicillin-resistant Staphylococcus aureus (MRSA). In studying the mupirocin biosynthetic cluster the authors identified two putative regulatory genes, mupR and mupl, whose predicted amino acid sequences showed significant identity to proteins involved in quorum-sensing-dependent regulatory systems such as LasR/LuxR (transcriptional activators) and LasI/Luxl (synthases for N-acylhomoserine lactones - AHLs - that activate LasR/LuxR). Inactivation by deletion mutations using a suicide vector strategy confirmed the requirement for both genes in mupirocin biosynthesis. Cross-feeding experiments between bacterial strains as well as solvent extraction showed that, as predicted, wild-type P. fluorescens NCIMB 10586 produces a diffusible substance that overcomes the defect of a mupl mutant. Use of biosensor strains showed that the Mupl product can activate the Pseudomonas aeruginosa lasRlasl system and that P. aeruginosa produces one or more compounds that can replace the Mupl product. Insertion of a xylE reporter gene into mupA, the first ORF of the mupirocin biosynthetic operon, showed that together mupR/mupl control expression of the operon in such a way that the cluster is switched on late in exponential phase and in stationary phase.