Regulation of methionine biosynthesis genes by quorum sensing in Burkholderia glumae

Abstract

Burkholderia glumae is a Gram-negative bacterium that produces a broad-spectrum phytotoxin, called toxoflavin. Toxoflavin production is dependent on the TofI/TofR quorum sensing system, which is mediated by N-octanoyl homoserine lactone (C8-HSL). C8-HSL is biosynthesized by TofI and its cognate receptor is TofR. The TofR/C8-HSL complex activates the expression of toxoflavin biosynthesis and an IclR-type transcriptional regulator gene, qsmR. Quorum sensing is a cell-to-cell signaling mechanism that refers to the ability of bacteria to respond to small diffusible autoinducers. When an autoinducer reaches a critical threshold, the bacteria detect and respond to the signals to alter gene expressions. Autoinducer is a byproduct of the activated methyl cycle (AMC), which is a central metabolic pathway responsible for the methylation of cellular components and the recycling of certain sulfur-containing amino acids. The last step of AMC involves the methylation of homocysteine to produce methionine. In Escherichia coli, there are two different enzymes, MetH and MetE, that catalyze methionine biosynthesis. MetH is dependent on vitamin B12 whereas MetE is not. In B. glumae, there are two metE homologs, metE1 and metE2, and a metH gene is split into two genes, metH1 and metH2. The objectives of this study are to determine if methionine biosynthesis is regulated by quorum sensing and identify regulation mechanisms of quorum sensing-dependent methionine biosynthesis genes in B. glumae. The metE1, metE2, or metH1-2 mutant showed similar growth rates as the wild type strain BGR1 in M9 minimal medium supplemented with 0.2% glucose. The metE1, metE2, and metH1-2 triple mutant was a methionine auxotroph. Autoinducer assay using a Chromobacterium violaceum biosensor on TLC plates indicated that there was no significant difference in C8-HSL production between the methionine biosynthesis gene mutants and the wild-type strain. Vitamin B12 bioassay provided indirect evidence for the vitamin B12 biosynthesis in B. glumae. Expression of metE1 is up-regulated by quorum sensing as proved by reverse transcription and real-time PCR experiments. Expression of metE2 was not expressed at a detectable level

however, its expression was activated in the absence of metH in a QS-dependent manner. These results indicate that methionine biosynthesis is controlled by quorum sensing

however, its biological meaning is still under investigation.