Quorum sensing-dependent proteomics and anticipation of stationary-phase stress in Burkholderia glumae

Abstract

Burkholderia glumae, the causative agent for bacterial rice grain rot, has a LuxR-LuxI type quorum sensing (QS) system. The bacterium utilizes N-octanoyl homoserine lactone synthesized by TofI and its cognate receptor TofR to activate expression of genes for toxoflavin biosynthesis and an IclR-type transcriptional regulator gene, qsmR. Since QS is essential for pathogenicity of B. glumae, we analyzed QS-dependent proteome by two-dimensional gel electrophoresis and ESI-MS/MS. We found that a total of 79 proteins, including previously known QS-depedent proteins, were differentially expressed between the wild-type BGR1 and the tofI mutant BGS2 strains. Among this set, 59 proteins were found in the extracellular fraction, and 20 were cytoplasmic. The extracellular 34 proteins including protease, lipase, phosphatases, were secreted through the type II secretion system (T2SS). Real-time RT-PCR analysis showed that the corresponding genes of the 49 extracellular and 13 intracellular proteins are regulated by QS at the transcriptional level. The T2SS, encoded by 12 general secretion pathway (gsp) genes with three independent transcriptional units, was controlled by QS. β-Glucuronidase activity analysis of gsp::Tn3-gusA gene fusions and electrophoretic mobility shift assays revealed that the QsmR directly regulates the expression of gsp genes. The T2SS defective mutants were less virulent than the wild type in rice panicles, indicating that the T2SS-dependent extracellular proteins play important roles in B. glumae virulence. Acyl-homoserine lactone-mediated QS regulates diverse activities in many species of Proteobacteria. QS-controlled genes commonly code for production of secreted or excreted public goods. QS affords a means of population density-dependent gene regulation. Control of public goods via QS provides a fitness benefit. Another potential role for QS is to anticipate overcrowding. As population density increases and stationary phase approaches, QS might induce functions important for existence in stationary phase. Here we provide evidence that in two related species of the genus Burkholderia QS allows individuals to anticipate and survive stationary phase stress, base toxicity. Survival requires QS-dependent activation of cellular enzymes required for production of excreted oxalate, which serves to counteract ammonia-mediated alkaline toxicity during stationary phase. Our findings provide an example where QS can serve as a means to anticipate stationary phase or life at the carrying capacity of the population by activating expression of cytoplasmic enzymes, altering cellular metabolism and producing a shared resource or public good, oxalate.