Dasgupta Nandini, Lykken Guinevere L, Wolfgang Matthew C, Yahr Timothy L
Department of Microbiology, W. M. Keck Microbial Communities and Cell Signaling Program, University of Iowa, 540B Eckstein Medical Research Building, Iowa City, IA 52242, USA.
Mol Microbiol. 2004 Jul;53(1):297-308. doi: 10.1111/j.1365-2958.2004.04128.x.
Expression of the Pseudomonas aeruginosa type III secretion system (TTSS) is coupled to the secretion status of the cells. Environmental signals such as calcium depletion activate the type III secretion channel and, as a consequence, type III gene transcription is derepressed. Two proteins, ExsA and ExsD, were shown previously to play a role in coupling transcription to secretion. ExsA is an activator of TTSS gene transcription, and ExsD is an anti-activator of ExsA. In the absence of environmental secretion cues, ExsD binds ExsA and inhibits transcription. Here, we describe the characterization of ExsC as an anti-anti-activator of TTSS expression. Transcription of the TTSS is repressed in an exsC mutant and is derepressed upon ExsC overexpression. The dependence on exsC for transcription is relieved in the absence of exsD, suggesting that ExsC and ExsD function together to regulate transcription. Consistent with this idea, ExsC interacts with ExsD in bacterial two-hybrid and co-purification assays. We propose a model in which the anti-anti-activator (ExsC) binds to and sequesters the anti-activator (ExsD) under low Ca(2+) conditions, freeing ExsA and allowing for transcription of the TTSS. The P. aeruginosa system represents the first example of an anti-activator/anti-anti-activator pair controlling transcription of a TTSS.
铜绿假单胞菌III型分泌系统（TTSS）的表达与细胞的分泌状态相关联。诸如钙耗竭等环境信号会激活III型分泌通道，结果导致III型基因转录被解除抑制。先前已证明两种蛋白质ExsA和ExsD在转录与分泌的偶联中发挥作用。ExsA是TTSS基因转录的激活剂，而ExsD是ExsA的反激活剂。在没有环境分泌信号的情况下，ExsD与ExsA结合并抑制转录。在此，我们描述了ExsC作为TTSS表达的抗反激活剂的特性。在exsC突变体中TTSS的转录受到抑制，而在ExsC过表达时则被解除抑制。在没有exsD的情况下，转录对exsC的依赖性得以缓解，这表明ExsC和ExsD共同发挥作用来调节转录。与这一观点一致，在细菌双杂交和共纯化实验中ExsC与ExsD相互作用。我们提出了一个模型，即在低钙（2+）条件下，抗反激活剂（ExsC）与反激活剂（ExsD）结合并使其隔离，释放出ExsA并允许TTSS转录。铜绿假单胞菌系统代表了控制TTSS转录的反激活剂/抗反激活剂对的首个实例。
