# 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  
While horizontal acquisition of pathogenicity islands such as the T-PAI is a common occurrence in the evolution of bacterial pathogens, additional evolutionary events are often needed to integrate newly acquired genetic material into existing regulatory networks [111; 112; 113]. It is worth noting that T3 SS regulators HrpR and HrpS belong to the NtrC-like enhancer-binding protein family, yet lack a canonical receiver domain and are unlikely to directly perceive T3 SS-inducing signals [56; 114]. Furthermore, proteins predicted to function as receptors for extracellular signals are not encoded within the T-PAI. Therefore, environmental regulation of the _P. syringae_ T3 SS is likely due to factors encoded outside of the T-PAI. As described in more detail below, the emerging picture is that host signal-dependent regulation of type III secretion is complex, with multiple independent signaling pathways that sense and respond to distinct host signals acting in combinatorial fashion to effect T3 SS deployment. Although how _P. syringae_ perceives T3 SS-inducing environmental signals is still poorly understood compared to other aspects of _P. syringae_ pathogenesis, in this section we highlight recent progress in identifying signaling components that link distinct environmental signals directly to the regulation of the canonical HrpR/HrpS-HrpL cascade.  
AauS-AauR Directly Regulates HrpRS Expression in P. syringae in Response to Specific Host-Derived Amino Acids  
Two-component systems are commonly involved in the environmental regulation of bacterial virulence [115; 116]. These regulatory systems consist of a sensor histidine kinase, often localized to the bacterial membrane, and an intracellular response regulator that functions as a transcription factor to modulate gene expression in response to the activating stimulus [117]. In a recent Tn5 genetic screen in _P. syringae_ pv. tomato DC3000, genes encoding the A\(\cidc  
module are genetically separable. To determine how the _aat_/_aau_ locus regulates T3 SS, the authors searched the DC3000 genome for Rbm sequences and identified an additional Rbm upstream of _lrprRS_, suggesting possible direct regulation of _hrprRS_ expression by AauR (Figure 2). Indeed, deleting the Rbm upstream of _hrprRS_ resulted in decreased T3 SS expression in response to aspartic acid and glutamate acid, as well as decreased DC3000 virulence on Arabidopsis [118]. The Rbm in the _hrprRS_ promoter is conserved across 38 isolates of _P. syringae_ containing the canonical T-PAI, indicating broad functional conservation of AauSR regulation of _hrprRS_. Consistent with this observation, _aauR_ in _P. syringae_ pv. _syritage_ B728a was also required for maximal T3 SS expression and virulence of B728a on host bean plants [118]. Based on these collective data, the authors hypothesized that the acquisition of Rbm upstream of _hrprRS_ occurred early in _P. syringae_ evolution as a means to connect the perception of environmental signals to the regulation of genes within the T-PAI. Given that aspartic acid and glutamic acid are among the most abundant amino acids in plants [84], the integration of T3 SS expression with a pre-existing mechanism of amino acid sensing was possibly a critical early step in the evolution of _P. syringae_ virulence.