Bacon Schneider Katherine, Palmer Tanya M, Grossman Alan D
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Bacteriol. 2002 Jan;184(2):410-9. doi: 10.1128/JB.184.2.410-419.2002.
Many microbes use secreted peptide-signaling molecules to stimulate changes in gene expression in response to high population density, a process called quorum sensing. ComX pheromone is a modified 10-amino-acid peptide used by Bacillus subtilis to modulate changes in gene expression in response to crowding. comQ and comX are required for production of ComX pheromone. We found that accumulation of ComX pheromone in culture supernatant paralleled cell growth, indicating that there was no autoinduction of production of ComX pheromone. We overexpressed comQ and comX separately and together and found that overexpression of comX alone was sufficient to cause an increase in production of ComX pheromone and early induction of a quorum-responsive promoter. These results indicate that the extracellular concentration of ComX pheromone plays a major role in determining the timing of the quorum response and that expression of comX is limiting for production of ComX pheromone. We made alanine substitutions in the residues that comprise the peptide backbone of ComX pheromone. Analysis of these mutants highlighted the importance of the modification for ComX pheromone function and identified three residues (T50, G54, and D55) that are unlikely to interact with proteins involved in production of or response to ComX pheromone. We have also identified and mutated a putative isoprenoid binding domain of ComQ. Mutations in this domain eliminated production of ComX pheromone, consistent with the hypothesis that ComQ is involved in modifying ComX pheromone and that the modification is likely to be an isoprenoid.
许多微生物利用分泌的肽信号分子来刺激基因表达的变化，以应对高种群密度，这一过程称为群体感应。ComX信息素是一种经过修饰的10氨基酸肽，枯草芽孢杆菌用它来调节基因表达的变化以应对拥挤。ComQ和ComX是产生ComX信息素所必需的。我们发现ComX信息素在培养上清液中的积累与细胞生长平行，这表明ComX信息素的产生不存在自诱导现象。我们分别或共同过表达comQ和comX，发现单独过表达comX就足以导致ComX信息素产量增加和群体感应响应启动子提前诱导。这些结果表明，ComX信息素的细胞外浓度在决定群体感应响应的时机方面起主要作用，并且comX的表达对ComX信息素的产生具有限制作用。我们对构成ComX信息素肽主链的残基进行了丙氨酸替换。对这些突变体的分析突出了这种修饰对ComX信息素功能的重要性，并确定了三个不太可能与参与ComX信息素产生或响应的蛋白质相互作用的残基（T50、G54和D55）。我们还鉴定并突变了ComQ的一个假定的类异戊二烯结合结构域。该结构域的突变消除了ComX信息素的产生，这与ComQ参与修饰ComX信息素且这种修饰可能是类异戊二烯的假设一致。
