ScmR, a global regulator of gene expression, quorum sensing, pH homeostasis, and virulence inBurkholderia thailandensis

Abstract

AbstractThe nonpathogenic soil saprophyteBurkholderia thailandensisis a member of theBurkholderia pseudomallei-thailandensis-mallei(Bptm) group, which also comprises the closely related human pathogensBurkholderia pseudomalleiandBurkholderia malleiresponsible for the diseases melioidosis and glanders, respectively. ScmR, a recently identified LysR-type transcriptional regulator (LTTR) inB. thailandensisacts as a global transcriptional regulator throughout the stationary phase, and modulates the production of a wide range of secondary metabolites, includingN-acyl-L-homoserine lactones (AHLs) and 4-hydroxy-3-methyl-2-alkylquinoline (HMAQ), virulence in the model hostCaenorhabditis elegans, as well as several quorum sensing (QS)-dependent phenotypes. We have investigated the role of ScmR inB. thailandensisstrain E264 during the exponential phase. We used RNA-Sequencing (RNA-Seq) transcriptomic analyses to identify the ScmR regulon, which was compared to the QS-controlled regulon, showing a considerable overlap between the ScmR-regulated genes and those controlled by QS. We characterized several genes modulated by ScmR, using quantitative reverse transcription-PCR (qRT-PCR) or mini-CTX-luxtranscriptional reporters, including the oxalate biosynthetic geneobc1required for pH homeostasis, the orphan LuxR-type transcriptional regulator BtaR5-encoding gene, thebsa(Burkholderiasecretion apparatus) type III secretion system (T3SS) genes essential for bothB. pseudomalleiandB. malleipathogenicity, as well as thescmRgene itself. We confirmed that the transcription ofscmRis under QS control, presumably ensuring fine-tuned modulation of gene expression. Finally, we demonstrate that ScmR influences virulence using the fruit fly model hostDrosophila melanogaster. We conclude that ScmR represents a central component of theB. thailandensisQS regulatory network.ImportanceCoordination of the expression of genes associated with bacterial virulence and environmental adaptation is often dependent on quorum sensing (QS). The QS circuitry of the nonpathogenic bacteriumBurkholderia thailandensis, which is widely used as a model system for the study of the human pathogenBurkholderia pseudomallei, is complex. We found that the recently identified LysR-type transcriptional regulator (LTTR), ScmR, which is highly conserved and involved in the control of virulence/survival factors in theBurkholderiagenus, is a global regulator mediating gene expression through the multiple QS systems coexisting inB. thailandensis, as well as independently of QS. We conclude that ScmR represents a key QS modulatory network element, ensuring tight regulation of the transcription of QS-controlled genes, particularly those required for acclimatization to the environment.