Burkholderia thailandensis harbors two identical rhl gene clusters responsible for the biosynthesis of rhamnolipids

Danielle Dubeau,Eric Déziel,Donald E Woods,François Lépine

doi:10.1186/1471-2180-9-263

Abstract

BackgroundRhamnolipids are surface active molecules composed of rhamnose and β-hydroxydecanoic acid. These biosurfactants are produced mainly by Pseudomonas aeruginosa and have been thoroughly investigated since their early discovery. Recently, they have attracted renewed attention because of their involvement in various multicellular behaviors. Despite this high interest, only very few studies have focused on the production of rhamnolipids by Burkholderia species.ResultsOrthologs of rhlA, rhlB and rhlC, which are responsible for the biosynthesis of rhamnolipids in P. aeruginosa, have been found in the non-infectious Burkholderia thailandensis, as well as in the genetically similar important pathogen B. pseudomallei. In contrast to P. aeruginosa, both Burkholderia species contain these three genes necessary for rhamnolipid production within a single gene cluster. Furthermore, two identical, paralogous copies of this gene cluster are found on the second chromosome of these bacteria. Both Burkholderia spp. produce rhamnolipids containing 3-hydroxy fatty acid moieties with longer side chains than those described for P. aeruginosa. Additionally, the rhamnolipids produced by B. thailandensis contain a much larger proportion of dirhamnolipids versus monorhamnolipids when compared to P. aeruginosa. The rhamnolipids produced by B. thailandensis reduce the surface tension of water to 42 mN/m while displaying a critical micelle concentration value of 225 mg/L. Separate mutations in both rhlA alleles, which are responsible for the synthesis of the rhamnolipid precursor 3-(3-hydroxyalkanoyloxy)alkanoic acid, prove that both copies of the rhl gene cluster are functional, but one contributes more to the total production than the other. Finally, a double ΔrhlA mutant that is completely devoid of rhamnolipid production is incapable of swarming motility, showing that both gene clusters contribute to this phenotype.ConclusionsCollectively, these results add another Burkholderia species to the list of bacteria able to produce rhamnolipids and this, by the means of two identical functional gene clusters. Our results also demonstrate the very impressive tensio-active properties these long-chain rhamnolipids possess in comparison to the well-studied short-chain ones from P. aeruginosa.

Highlights

Rhamnolipids are surface active molecules composed of rhamnose and βhydroxydecanoic acid
Presence of rhlABC homologs in B. thailandensis and B. pseudomallei Following a nucleotide and protein similarity search using algorithms blastn and blastp with standard parameters http://blast.ncbi.nlm.nih.gov/Blast.cgi, respectively, in sequenced B. thailandensis and B. pseudomallei genome sequences, close orthologs of the P. aeruginosa rhamnolipid-biosynthesis genes rhlA, rhlB and rhlC were found in all associated strains as gene clusters
The protein encoded by these genes share almost 50% identity with those of P. aeruginosa PAO1, which possesses a single copy of these genes on its genome

Summary

Introduction

Rhamnolipids are surface active molecules composed of rhamnose and βhydroxydecanoic acid These biosurfactants are produced mainly by Pseudomonas aeruginosa and have been thoroughly investigated since their early discovery. They have attracted renewed attention because of their involvement in various multicellular behaviors. Rhamnolipids are surface-active compounds that have been extensively studied since their early identification in Pseudomonas aeruginosa cultures in the late 1940s [1]. It was only in the mid 1960s that the structure of a rhamnolipid molecule was first reported [2]. Rhamnolipids inhibit the phagocytic response of macrophages and are known as the heat-stable extracellular hemolysin produced by P. aeruginosa [11,12]

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