# Regulation of the _Pseudomonas syringae_ Type III Secretion System by Host Environment Signals
## 3 Molecular Mechanisms of T3 SS Deployment in Response to Environmental Stimuli
### The DeoR-Type Regulator SetA Is Necessary for _hrpl Expression in Response to Sugar Signals_  
Simple sugars such as sucrose and glucose are abundant plant metabolites present within the leaf apoplast and exuded onto leaf surfaces [83; 84]. As such, they are likely among the first metabolites encountered by _P. syringae_ during infection. During culturing of _P. syringae_, the addition of a simple sugar to the culture medium is sufficient to induce T3 SS gene expression [83]. Furthermore, maximal activation of T3 SS genes by organic acids and amino acids in plant exudates, as described in Section 2.2 above, also requires the presence of sugars in the induction medium. Together, these observations suggest sugars may be important signals for _P. syringae_ to initiate infection. In a recent Tn5 mutagenesis screen, two _P. syringae_ DC3000 mutants were recovered that contained insertion mutations in a gene encoding SetA (for Sugar-Induced Expression of T3 SS A), a predicted DeoR-type transcription factor [121]. Phenotypic analysis of these mutants revealed that SetA was partially required for DC3000 expression of _hrpl_ and effector _avrRpm1_ in response to multiple sugars and sugar alcohols such as sucrose, fructose and mannitol, as well as  
Figure 2: Model of regulation of the T3 SS in _P. syringae_ by AauSR. Depicted is a _P. syringae_ cell, with only the inner membrane (separating the periplasm and cytoplasm) shown for clarity. In the periplasm, acidic amino acids such as aspartic and glutamic acid are bound by periplasmic solute binding protein AatJ (mauve). AatJ assists transport of these amino acids into the cytoplasm through interaction with the AatQMP ABC transporter (pink), and may additionally interact with the inner membrane histidine kinase AauS (teal). AauS activates its cognate response regulator AauR (olive) through phosphorlex. In its active conformation, AauR stimulates expression of the _aatJ_/_QMP_ operon through binding of a conserved AauR binding motif (Rbm) in the _aatJ_ promoter. AauR additionally induces _hrprRS_ expression through binding an Rbm in the _hrprRS_ promoter. HrpRS (aqua) then bind the enhancer of T3 SS master regulator _hrpl_, leading to activation of the T3 SS cascade. Figure was made using BioRender ([[https://app.biorender.com](https://app.biorender.com)]([https://app.biorender.com](https://app.biorender.com)), accessed on 30 April2021).  
maximal _hrpl_ expression within the leaves of host Arabidopsis plants [121]. In this study, a DC3000 _setA_:Tn5 mutant was unable to grow to high levels and elicit disease symptoms on infected host plants, likely due to impaired activation of its T3 SS. Although _hrpl_ expression was compromised in DC3000 _setA_:Tn5, the expression of upstream regulators _hrpRS_ and _rpoN_ was not altered, suggesting SetA may function at the level of _hrpl_ expression or post-transcriptional regulation of HrpRS and/or RpoN [121]. Given that DeoR-type proteins typically function as DNA-binding transcriptional repressors, SetA may function by inhibiting the expression of a negative regulator of _hrpl_. In addition to its DNA-binding domain, SetA also possesses a C-terminal sensor domain that, based on other DeoR-type regulators, likely binds specific intracellular metabolites and regulates the DNA-binding activity of SetA. While sugar-induced expression of _hrpl_ was partially SetA-dependent, DC3000 growth in media containing sugars as a sole carbon source did not require SetA, indicating that sugar-induced expression of the T3 SS is genetically separable from sugar catabolism. These observations suggest SetA may function in regulating T3 SS genes by sensing and responding to the intracellular accumulation of sugars or associated downstream catabolic products. Further characterization is necessary to identify the targets of SetA transcriptional regulation, as well as ligands that may bind to and module SetA activity.