Department of Molecular Microbiology and Immunology, Oregon Health and Science University, Portland, Oregon, USA.
J Bacteriol. 2012 Apr;194(8):1897-911. doi: 10.1128/JB.06614-11. Epub 2012 Feb 3.
TonB systems transduce the proton motive force of the cytoplasmic membrane to energize substrate transport through a specific TonB-dependent transporter across the outer membrane. Vibrio vulnificus, an opportunistic marine pathogen that can cause a fatal septicemic disease in humans and eels, possesses three TonB systems. While the TonB1 and TonB2 systems are iron regulated, the TonB3 system is induced when the bacterium grows in human serum. In this work we have determined the essential roles of the leucine-responsive protein (Lrp) and cyclic AMP (cAMP) receptor protein (CRP) in the transcriptional activation of this system. Whereas Lrp shows at least four very distinctive DNA binding regions spread out from position -59 to -509, cAMP-CRP binds exclusively in a region centered at position -122.5 from the start point of the transcription. Our results suggest that both proteins bind simultaneously to the region closer to the RNA polymerase binding site. Importantly, we report that the TonB3 system is induced not only by serum but also during growth in minimal medium with glycerol as the sole carbon source and low concentrations of Casamino Acids. In addition to catabolite repression by glucose, l-leucine acts by inhibiting the binding of Lrp to the promoter region, hence preventing transcription of the TonB3 operon. Thus, this TonB system is under the direct control of two global regulators that can integrate different environmental signals (i.e., glucose starvation and the transition between "feast" and "famine"). These results shed light on new mechanisms of regulation for a TonB system that could be widespread in other organisms.
TonB 系统将细胞质膜的质子动力转化为能量，通过特定的 TonB 依赖性转运蛋白将底物转运穿过外膜。创伤弧菌是一种机会性病原体，可导致人类和鳗鱼致命的败血症，它具有三个 TonB 系统。虽然 TonB1 和 TonB2 系统受铁调节，但当细菌在人血清中生长时，TonB3 系统会被诱导。在这项工作中，我们确定了亮氨酸反应蛋白 (Lrp) 和环腺苷酸 (cAMP) 受体蛋白 (CRP) 在该系统转录激活中的必需作用。虽然 Lrp 至少有四个非常独特的 DNA 结合区域从位置 -59 到 -509 展开，但 cAMP-CRP 仅结合在从转录起始点开始的位置 -122.5 处的中心区域。我们的结果表明，这两种蛋白同时结合到更接近 RNA 聚合酶结合位点的区域。重要的是，我们报告说，TonB3 系统不仅被血清诱导，而且在以甘油为唯一碳源和低浓度 Casamino Acids 的最小培养基中生长时也被诱导。除了葡萄糖的分解代谢抑制外，l-亮氨酸通过抑制 Lrp 与启动子区域的结合来起作用，从而阻止 TonB3 操纵子的转录。因此，该 TonB 系统受两个全局调节剂的直接控制，这两个调节剂可以整合不同的环境信号（即葡萄糖饥饿和“饱食”与“饥饿”之间的转变）。这些结果为 TonB 系统的新调控机制提供了启示，该机制可能在其他生物体中广泛存在。
