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Chapter 13 : Role of Quorum-Sensing Regulation in Pathogenesis of Pantoea stewartii subsp. stewartii
Category: Microbial Genetics and Molecular Biology; Bacterial Pathogenesis
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A glimpse into the disease biology of Pantoea stewartii comes from early biochemical and ultra-structural studies of the organism in culture and in the infected maize tissues. The promoter region of esaR features a wellconserved lux box-like element, the esaR box, which spans a putative -10 hexameric σ70 promoter element. The authors focused initially on this promoter to define the functional role of EsaR as a transcription factor and DNA-binding protein. EsaR showed functional attributes that were essentially the reverse from that of the LuxR and TraR functional paradigms and other orthologous proteins that are acyl homoserine lactone (AHL)-dependent quorum-sensing activators. The observation that P. stewartii expresses the major EPSST virulence factor in a cell-density-dependent manner suggests a key role for the EsaI/EsaR quorum-sensing system in managing the transition between distinct phases of bacterial/biofilm development, which may be key to pathogen fitness during host colonization. In addition, other surface localized functions will be characterized to identify components that initiate the contact between the bacterium and the xylem wall, as seen in infections with the AHL mutant strain ESN51. Finally, it might be of interest to know that P. stewartii was initially selected as an experimental organism for quorum-sensing control because of its capacity to synthesize high amounts of AHL.
Key Concept Ranking
- Type III Secretion System
Stewart’s wilt. (A) Typical parallel streaking symptoms due to pathogen colonization of parallel longitudinal maize xylem. (B) Beetle vector feeding on a maize leaf. Note the feeding scars (arrow).
Genes and proteins necessary for EPSST synthesis and its repeating unit structure. (A) Schematic depiction of gene systems proven and predicted to contribute to Stewartan capsular/exopolysaccharide synthesis. The primary wce gene system features 12 genes beginning with wceG at the 5′ end and ending with wzx1 at the 3′ end. The wceG1 gene is preceded by the primary RcsA/B-regulated promoter for induced expression of the system (➨). Two internal promoters are located upstream of wza and wceB. These promoters appear to be constitutively expressed (➙). Two putative attenuator loops (Ϟ) bracket wceL. The wceG2 gene, which appears to be constitutively expressed, is functionally equivalent to the RcsA/B-regulated wceG1 (A. L. Carlier and S. B. von Bodman, unpublished data). A separate two-gene system, encoding wceO and wzx2, is also RcsA/B regulated (A. L. Carlier and S. B. von Bodman, unpublished data). (B). List of confirmed and predicted products of the CPSST/EPSST biosynthetic genes. The roman numerals to the right of specific genes correspond to the residue designations of the heptasaccharide structure in panel C. The hexose sugar linkages are as indicated. Polymerization between adjacent repeating units involves a β1-3 linkage between Gal residues I and III.
Model of the converging EsaI/EsaR quorum-sensing and Rcs environmental stimuli-sensing regulatory networks. The dominant quorum-sensing cell-cell signaling pathway involves the EsaR transcription factor and cognate AHL signal synthase, EsaI. The two genes are genetically linked, encoding two convergent, slightly overlapping open reading frames. The esaI gene is constitutively expressed, leading to linear accumulation of AHL (⌂). EsaR represses its own gene expression and blocks the RcsA-dependent transcription of rcsA under subthreshold AHL levels. The AHL-free form of EsaR binds DNA and represses and activates target genes. DNA-binding and transcriptional activity of Apo-EsaR is neutralized by inducing levels of AHL. Derepression leads to the RcsA/B-dependent activation of rcsA, which represents a positive feedback regulatory loop. RcsA is a key component of the Rcs environmental signal sensing ( ) phosphorelay system, which includes the outer membrane lipoprotein RcsF. RcsF perceives and transmits undefined environmental signals and membrane pertubation stimuli to the inner membrane-bound RcsC sensor kinase with a receiver domain that passes the phosphoryl group to RcsD (formerly YojN). RcsD is also an inner membrane-bound phosphotransfer protein involved in transferring the phosphoryl group to the RcsB response regulator. RcsB has multiple functions but, if complexed with the RcsA coactivator, is dedicated for wce activation. RcsA/B-activated (➙) promoters include the primary promoter of the wce gene system and the promoter upstream of wceO/wzx2. Their coordinate activation leads to increased production of the heptasaccharide repeating units required for synthesis of EPSST (■). The promoters (→) upstream of wceB and wceG2 are thought to facilitate low-level constitutive repeat unit synthesis for CPSST (TTT), possible K-LPS synthesis.
P. stewartii colonization of xylem wall structures visualized by SEM. (A) An annular ring colonized by the wild-type strain DC283. The black arrow indicates an extensive, gummy strandlike network that covers the annular ring surfaces in heavily infected tissue. (B) These fibrils are less numerous in sparsely colonized areas observed at higher magnification. (C) Annular ring excised from an uninfected xylem vessel. (D) The AHL, EPSST-deficient strain, ESN51, also colonizes the annular rings. Note the absence of the gummy strands and the presence of undefined appendages that mediate adhesion to the cell wall, as indicated by the black arrow (E).