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

Abstract

The nonpathogenic soil saprophyte Burkholderia thailandensis is a member of the Burkholderia pseudomallei-thailandensis-mallei (Bptm) group, which also comprises the closely related human pathogens Burkholderia pseudomallei and Burkholderia mallei responsible for the diseases melioidosis and glanders, respectively. ScmR, a recently identified LysR-type transcriptional regulator (LTTR) in B. thailandensis acts as a global transcriptional regulator throughout the stationary phase, and modulates the production of a wide range of secondary metabolites, including N-acyl-L-homoserine lactones (AHLs) and 4-hydroxy-3-methyl-2-alkylquinoline (HMAQ), virulence in the model host Caenorhabditis elegans, as well as several quorum sensing (QS)-dependent phenotypes. We have investigated the role of ScmR in B. thailandensis strain 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-lux transcriptional reporters, including the oxalate biosynthetic gene obc1 required for pH homeostasis, the orphan LuxR-type transcriptional regulator BtaR5-encoding gene, the bsa (Burkholderia secretion apparatus) type III secretion system (T3SS) genes essential for both B. pseudomallei and B. mallei pathogenicity, as well as the scmR gene itself. We confirmed that the transcription of scmR is under QS control, presumably ensuring fine-tuned modulation of gene expression. Finally, we demonstrate that ScmR influences virulence using the fruit fly model host Drosophila melanogaster. We conclude that ScmR represents a central component of the B. thailandensis QS 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 bacterium Burkholderia thailandensis, which is widely used as a model system for the study of the human pathogen Burkholderia 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 the Burkholderia genus, is a global regulator mediating gene expression through the multiple QS systems coexisting in B. 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.