A secreted regulatory protein couples transcription to the secretory activity of the Pseudomonas aeruginosa type III secretion system

Abstract

The type III secretion system (T3SS) of Pseudomonas aeruginosa is an important virulence determinant. Transcription of the T3SS is highly regulated and intimately coupled to the activity of the type III secretion channel. The secretion channel is generally closed, and transcription is repressed. Inducing signals such as calcium depletion, however, open the secretion channel and derepress transcription of the T3SS. The coupling of transcription with secretion requires three previously identified cytoplasmic regulatory proteins. ExsA is a DNA-binding protein required for transcriptional activation of the entire T3SS. The second regulatory protein, ExsD, functions as anti-activator by directly binding to ExsA. Finally, ExsC functions as an anti-anti-activator by directly binding to and inhibiting ExsD. Although the regulatory roles of ExsC, ExsD, and ExsA were defined through these previous studies, the mechanism of coupling transcription to secretion was unclear. We now report the identification of ExsE as a secreted regulator of the T3SS and provide evidence that ExsE functions as a direct inhibitor of ExsC. When the secretion channel is closed, ExsE is complexed with ExsC in the cytoplasm, and transcription of the T3SS is repressed by sequestration of ExsA by ExsD. We propose that the secretion of ExsE provides an initiating signal that results in an equilibrium shift whereby ExsC becomes preferentially bound to ExsD, thus allowing liberated ExsA to activate transcription of the T3SS. The presence of ExsE homologs in the T3SSs of other bacterial species suggests that this mechanism of coupling transcription to secretion may be commonly used.anti-activator ͉ anti-anti-activator ͉ ExsE

Control of gene expression by type III secretory activity

“…ExsC functions as an anti-anti-activator by binding to and inhibiting the negative regulatory activity of ExsD [30]. ExsC also serves as a type III secretion chaperone for ExsE [31] [32]. Activation of the injectisome by host cell contact triggers translocation of ExsE resulting in a corresponding decrease in the cytoplasmic concentration of ExsE [33].…”

The response regulator SsrB activates expression of diverse Salmonella pathogenicity island 2 promoters and counters silencing by the nucleoid‐associated protein H‐NS

“…A feedback control model has been proposed whereby once SipB and SipC are secreted, SicA is free to interact with InvF to stimulate transcription of downstream effector genes (Miller, 2002). Distinct feedback systems have also been proposed for Yersinia and Pseudomonas species, but such a feedback system has not yet been reported for the SPI-2 TTSS (Francis et al, 2001;Dasgupta et al, 2004;Rietsch et al, 2005;Urbanowski et al, 2005).…”

Anti-activator ExsD Forms a 1:1 Complex with ExsA to Inhibit Transcription of Type III Secretion Operons

“…The anti-activator ExsD belongs to the dedicated regulatory cascade that links synthesis of T3SS proteins to secretory activity (21). This pathway involves also the anti-anti-activator ExsC (23) and the secreted/ translocated protein ExsE (24,25). Based on in vivo and in vitro studies, a model of regulation has been proposed (10): when the T3SS is induced, after direct contact with the target cell or calcium depletion, the small protein ExsE is translocated through the syringe-like apparatus into the host cell; consequently, ExsC binds ExsD, which releases ExsA that becomes able to activate transcription of the T3SS genes.…”