# Regulation of the _Pseudomonas syringae_ Type III Secretion System by Host Environment Signals
### Post-Translational Control of the P. syringae T3 SS by hrp/hrc Regulators  
Additional proteins encoded within the conserved _hrp/hrc_ pathogenicity cluster also contribute to the regulation of T3 SS deployment. HrpS, half of the HrpRS heterodimer that stimulates the transcription of _hrpL_, is subject to post-translational regulation by regulatory pair HrpV and HrpG. By the current model, HrpV directly binds to HrpS, thus limiting the ability of HrpS to bind to HrpR [36; 59; 64]. HrpV also binds to HrpG, a chaperone protein associated with the T3 SS basal body on the cytoplasmic face of the bacterial inner membrane [59; 65]. Under conditions where T3 SS expression is activated, HrpV is hypothesized to dock at the cell membrane through its interaction with HrpG, thus spatially preventing HrpV association with bacterial DNA and the repression of HrpS [66]. At the inner membrane, the HrpVG complex additionally binds to HrpJ, another regulatory protein encoded in the _hrp/hrc_ cluster. HrpJ is secreted through the T3 SS, and carries out an unknown function within plant cells to suppress the host immune response [67]. Prior to its own translocation, HrpJ functions to plug the T3 SS inner channel through the formation of a gatekeeper-like ternary complex with HrpVG [66]. The formation of this gatekeeper complex may occur via HrpG interaction with HrcU, an additional inner membrane-associated component of the T3 SS core [66]. HrpJ is necessary for full T3 SS function by _P. syringae_ in the host plant environment, and is required for the extracellular secretion of harpins and translocator proteins [67; 68; 69]. The occlusion of the T3 SS inner channel by the HrpVGJ ternary complex may gate the secretion of later T3 SS substrates, such as harpins and effectors, pending additional secretion-inducing signals.  
The HrpA1 pilus protein, described in Section 1.2 for its role as a positive regulator of the T3 SS cascade, is also subject to post-translational regulation. The HrpA1 protein is rapidly degraded within the bacterial cell but can be stabilized through binding by T3 SS regulatory protein HrpF [70; 71]. HrpF is critical for disease and the production of a functional T3 SS by various _P. syringae_ strains [37; 71], suggesting that HrpF stabilization of HrpA1 may be important for T3 SS biogenesis. HrpF has also been shown to function as a negative regulator of the T3 SS through interaction with HrpG [71]. Binding by HrpF may titrate HrpG from HrpG-HrpV complex formation, enabling HrpV to migrate away from the membrane and repress HrpS. The authors of this study proposed that the interaction of HrpF with HrpA1 during the early stages of T3 SS production may reduce HrpG-HrpF interaction while favoring HrpG-HrpV complex formation, de-repressing HrpS and amplifying the expression of the T3 SS signaling cascade. Further resolution of the HrpVGJ circuit, and its potential interplay with the intracellular dynamics of HrpF and/or HrpA1, will be a key part of defining the regulatory relationship between T3 SS expression and secretion.  
The HrpA1 protein also accumulates extracellularly prior to the secretion of other T3 SS substrates, including harpins and effector proteins [54; 70]. In one study, an increase in the copy number of _hrpL_ transcripts leads to increased extracellular accumulation of HrpA1 alone, while T3 SS harpin and effector proteins accumulate intracellularly but were not secreted in higher amounts [54]. These findings suggest that the induction of the T3 SS by _P. syringae_ occurs in two discrete phases: (1) HrpL-dependent expression of T3 SS genes and secretion of HrpA1, and (2) secretion of downstream T3 SS substrates, which is likely regulated at the post-translational level. The means by which downstream T3 SS substrate secretion is \"unlocked\" have not been identified, though the maturation of the HrpA1 pilus and eventual contact with the host plasma membrane may trigger secretion [70].