Identification of a quorum sensing-dependent two component system that confers pleiotropic phenotypes in Burkholderia glumae

다국어 초록 (Multilingual Abstract)

Burkholderia glumae is a Gram-negative bacterium, which causes grain rot (panicle bight) and wilt in rice. B. glumae produces a broad-host range phytotoxin, called toxoflavin that holds a key virulence factor in rice grain rot disease and bacterial wilt in many field crops. The biosynthesis of toxoflavin is regulated by quorum sensing, which is a cell to cell communication mechanism that bacteria count their own numbers by producing and detecting the accumulation of a signaling molecule that they export into environment. The accumulation of a signaling molecule is the LysR-type transcriptional activator ToxR. ToxR depends on N-octanoyl homoserine lactone (C8-HSL) that is biosynthesized by TofI and its cognate receptor is TofR. The TofR and C8-HSL complex activates qsmR expression by binding to the qsmR promoter region and help bacteria to respond to its environment.
In bacteria, most two-component signal transduction systems (TCS) are composed of a sensor histidine kinase and a response regulator. TCS is the predominant means by which bacteria sense and respond to extracellular signals to allow cells to adapt to its prevailing conditions by modifying initiating programs of gene expression.
In this study, a mutation in TCS transcriptional regulator gene (grtR) and phosphocarrier protein HPr gene (HPr) in B. glumae has been a focus to show the cell growth and morphogenesis are affected by quorum sensing dependent TCS. When TCS is defective (grtR mutant), along with a point mutation in HPr, the mutant colony resulted pleiotripic phenotypes and its morphology dramatically changed that its colonial surface became very glossy in B. glumae wild type background mutant. However, there was no pleiotropy accompanied by glossy colonial surface in HPr knock out mutants that they rather have had matte colonial surface. In the case of grtR and qsmR double mutant, there was very dense crystal formation indicating an ability to adjust its cytoplasmic pH in B. glumae with defective TCS and a mutation in citrate synthase gene.
Therefore, the above study may be concluded as follow: when TCS is defected in B. glumae, the mutant cannot regulate certain genes for the better fitness in their environment causing incorrect signal sensing and responding, hence resulting pleiotropic phenotypes for their survival.

국문 초록 (Abstract)

Burkholderia glumae는 세균밀도인식기작, quorum sensing (QS)에 의존하여 세균성 벼알마름병의 병원성, 독소(Toxoflavin)의 생합성과 분비, biofilm의 형성, flagella motility, 환경적응을 위한 two component system (TCS) 등을 조절한다. 본 연구에서는 B. glumae에서 QS의 조절을 받는 TCS중의 하나인 grtK와 grtR에 Tn3-gusA insertion으로 인한 grtR에 mutation이 일어나면 pleiotropic phenotype을 나타내는 것을 확인하였다. Pleiotropic phenotypes 중 흥미로운 표현형 두 가지 중 하나는 whole genome sequencing 결과에 의하면 QS 할 수 없는 mutant strain 에서 grtR에 Tn3-gusA insertion으로 인한 mutation과 citrate synthase에 spontaneous mutation으로 인해 QS를 할 수 있게 되어 mutant colony 안에 crystal을 합성이 가능해 지는 것 과 또 다른 하나의 표현형은QS가능한 strain에서 Tn3-gusA insertion으로 인한 grtR에 mutation 때문에glossy한 colony surface를 가지는 것으로써 whole genome sequencing 결과에 의하면 phosphocarrier protein HPr (HPr) 에 point mutation으로 인한 phenotype으로 확인되었다. 이를 토대로 지금까지 발견 되지 않았던glossy한 표현형을 중심으로 EZTn5 insertion으로 인한 HPr knock out mutant를 통해 glossy colonial surface phenotype을 확인 해보았으나 matte한 colonial surface를 가지는 phenotype이 발현되는 것을 확인 하였다. 따라서 TCS를 이루는 grtK와 grtR은 B. glumae의 환경 변화에 따른 적응에 필요한 유전자 발현 경로에 영향을 미치는 것으로 추정되었다.