AlgU Controls Expression of Virulence Genes in Pseudomonas syringae pv. tomato DC3000

Abstract

Plant-pathogenic bacteria are able to integrate information about their environment and adjust gene expression to provide adaptive functions. AlgU, an extracytoplasmic function (ECF) sigma factor encoded by Pseudomonas syringae, controls expression of genes for alginate biosynthesis and genes involved with resisting osmotic and oxidative stress. AlgU is active while these bacteria are associated with plants, where its presence supports bacterial growth and disease symptoms. We found that AlgU is an important virulence factor for P. syringae pv. tomato DC3000 but that alginate production is dispensable for disease in host plants. This implies that AlgU regulates additional genes that facilitate bacterial pathogenesis. We used transcriptome sequencing (RNA-seq) to characterize the AlgU regulon and chromatin immunoprecipitation sequencing (ChIP-seq) to identify AlgUregulated promoters associated with genes directly controlled by this sigma factor. We found that in addition to genes involved with alginate and osmotic and oxidative stress responses, AlgU regulates genes with known virulence functions, including components of the Hrp type III secretion system, virulence effectors, and the hrpL and hrpRS transcription regulators. These data suggest that P. syringae pv. tomato DC3000 has adapted to use signals that activate AlgU to induce expression of important virulence functions that facilitate survival and disease in plants. IMPORTANCEPlant immune systems produce antimicrobial and bacteriostatic conditions in response to bacterial infection. Plant-pathogenic bacteria are adapted to suppress and/or tolerate these conditions; however, the mechanisms controlling these bacterial systems are largely uncharacterized. The work presented here provides a mechanistic explanation for how P. syringae pv. tomato DC3000 coordinates expression of multiple genetic systems, including those dedicated to pathogenicity, in response to environmental conditions. This work demonstrates the scope of AlgU regulation in P. syringae pv. tomato DC3000 and characterizes the promoter sequence regulated by AlgU in these bacteria. Pseudomonas syringae is a globally dispersed plant-pathogenic bacterium adapted to live in many diverse environments, which in some cases expose bacteria to stresses that can interfere with their growth and survival. Counteracting or tolerating these stresses requires coordinated expression of specific physiologic traits. These responses are controlled by environmental sensing and signal transduction systems that detect information about external conditions and induce changes in transcription. The extracytoplasmic function (ECF) sigma factors are one type of system providing this function in bacteria and are capable of inducing sets of genes (regulons) in response to environmental stimuli (1). ECF sigma factor activity is often controlled by a cytoplasmic membrane-bound anti-sigma factor that forms a complex with the ECF sigma factor (2). Under inducing conditions, the ECF sigma factor is liberated from the anti-...

Ca 2+ -Induced Two-Component System CvsSR Regulates the Type III Secretion System and the Extracytoplasmic Function Sigma Factor AlgU in Pseudomonas syringae pv. tomato DC3000

“…tomato DC3000 (15). Interestingly, and somewhat contradictory to reported results showing that overexpression of algU increases virulence of P. syringae pv.…”

“…RNA-seq results showed a greater-than-2-fold increase in expression of algU, mucA, and mucB in the ΔcvsR strain. In addition, 94 genes previously reported to be part of the AlgU regulon are shared with the CvsR regulon (15). Of these 94 genes, 77 displayed the same expression profile in an algUoverexpressing strain and in the ΔcvsR strain compared to the WT.…”

“…Overexpression of algU in P. syringae pv. tomato DC3000 resulted in increased expression of cvsS and cvsR (15). Both cvsS and cvsR are directly regulated by CvsR.…”

“…CvsR regulates transcription of algU, and the regulons of CvsR and AlgU have noticeable overlap (15). It is currently unclear how CvsR transcriptionally regulates algU in P. syringae pv.…”

“…Ambiguous reads were removed as previously described (88). Next, a list of regions to be considered for differential analysis was constructed as previously described (15).…”