# Regulation of the _Pseudomonas syringae_ Type III Secretion System by Host Environment Signals
### Transcriptional Control of P. syringae T3 SS Genes by hrp/hrc Regulators  
The master regulator of the T3 SS is HrpL, an extracytoplasmic function (ECF) alternative sigma factor that interacts with a conserved \"_lirp_-box\" motif in the promoters of target genes (Figure 1B) [41; 42; 43]. By recruiting RNA polymerase, HrpL directs the transcription of the _lirp_/_hrc_ pathogenicity cluster as well as various T3 SS effector genes [44; 45; 46; 20; 21; 47; 48]. HrpL also regulates the expression of genes unrelated to T3 SS function, including those involved in various metabolic processes and the production of bacterial toxins relevant to _P. syringae_ pathogenesis [44; 48; 21; 49]. The resolution of the full HrpL regulon in _P. syringae_ was enabled by the availability of whole-genome sequences for various _P. syringae_ isolates, beginning with model strain _P. syringae_ pv. tomato DC3000 [50; 51; 52]. As the timing and magnitude of induction differ among HrpL-regulated genes, some authors have postulated that HrpL orchestrates the expression of the T3 SS-encoding genes in a hierarchical manner [48; 53; 54]. By this model, genes encoding the core regulatory and structural components of the T3 SS apparatus, such as the extracellular pilus protein HrpA1, are initially activated by HrpL, while genes encoding T3 SS effectors and secretion accessory proteins are transcribed secondarily [48]. A variety of genes unrelated to T3 SS function have also been identified. The transcription of _lirpL_ itself is directed by the \(\sigma^{54}\) sigma factor (RpoN) and the enhancer-binding proteins HrpR and HrpS [55; 56; 57; 43]. The genes encoding HrpR and HrpS are paired in a _lirpRS_ operon within the _lirp/hrc_ cluster [56; 58]. In _P. syringae_ pv. _phaselicola_ NPS3121, HrpR is required for _lirpS_ transcription, potentially through interaction with an upstream regulatory \"HrpR-box\" sequence [58]. HrpR and HrpS oligomerize to form a heterohexameric complex and together are required for full _lirpL_ gene expression, though HrpS is capable of functioning independently as a weak activator of the _lirpL_ promoter in  
Figure 1: Regulation of the T3 SS by components of the _lirp/hrc_ pathogenicity cluster in _P. syringae_. (**A**) Depicted is a schematic of the _lirp/hrc_ pathogenicity cluster within the tripartite T3 SS pathogenicity island in _P. syringae_ pv. _syngage_ 61. This schematic is adapted from [PERSON] et al., 2000 [34]. Colored boxes depict open reading frames, with corresponding _lirp/hrc_ gene names listed below. Genes encoding T3 SS master regulators HrpL and HrpRS are shaded in coral and teal, respectively. Shaded in violet are additional regulators of T3 SS expression that function at either the transcriptional or post-transcriptional level. (**B**) Depicted is a diagram of the T3 SS regulatory cascade in _P. syringae_. Genes encoding the regulatory proteins HrpR and HrpS (teal) are transcribed from the _lirpRS_ operon within the _lirp/hrc_ pathogenicity cluster. HrpRS oligomerize (depicted as a dimer for clarity) and bind the _lirpL_ promoter. Together with alternate sigma factor RpoN (gold), which recruits RNA polymerase (blue) to the _lirpL_ promoter, HrpRS stimulate transcription of _lirpL_. HrpL (coral) functions as an alternate sigma factor to recruit RNA polymerase to a conserved binding motif, termed the “hrp box,” in the promoters of target genes. By this mechanism, HrpL directs the transcription of downstream genes, including T3 SS-associated genes. 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). the absence of HrpR [56; 59]. In addition to activation of _hrpL_, HrpS directs the expression of a variety of T3 SS-independent genes throughout the _P. syringae_ genome [45; 60; 61]. A broad de-repression of housekeeping gene expression has been observed in _hrpL_: and _hrpRS_: loss-of-function mutants of _P. syringae_ pv.