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Chapter 16 : The hrp Cluster of Pseudomonas syringae: aPathogenicity Island Encoding a Type III Protein Translocation Complex?
This chapter focuses on the pathogenicity determinants of Pseudomonas syringae as a paradigm for microbial pathogenesis in plants. It focuses specifically upon genetic determinants of P. syringae strains necessary for colonization of plant tissue. As in mammalian pathogens, essential pathogenicity determinants of several plant-pathogenic bacteria are localized in apparent pathogenicity islands (PAIs). Proteins translocated by a type III protein translocation complex (PTC) similar to that of its mammalian counterparts mediate both the pathogenicity and host range of P. syringae strains. Hybridization analysis indicates that all P. syringae strains carry a homolog to HrpW and that a homolog is present in the closely related Erwinia hrp cluster. The pectate lyase-like domain was the most highly conserved region of the protein from two P. syringae strains. The genetic organization of clusters and regulatory mechanisms controlling environmental regulation are clearly distinct between the two groups of hrp clusters. For example, the primary regulatory factor for the X. campestris and R. solanacearum hrp clusters is an AraC homolog (HrpX and HrpB, respectively. By analogy to mammalian pathogens, pathogenesis by plant pathogenic bacteria, such as P. syringae, probably involves (i) adhesion of the bacteria to plant cells, (ii) activation of a type III PTC, (iii) translocation of pathogenicity determinants into plant cells, (iv) physiological changes in the host cells to stimulate the release of nutrients, (v) production of virulence factors to facilitate the growth of the bacteria, and (vi) growth and spread of the bacteria to surrounding cells in the tissue.
The hrp Cluster of Pseudomonas syringae: aPathogenicity Island Encoding a Type III Protein Translocation Complex?
Organization of the P. syringae hrp cluster. Shaded polygons or boxes represent genes. The arrowhead indicates the deduced direction of transcription for the operon. Operon designations are indicated below the figure, and gene designations are given above their respective component. Gene designations are segregated into hrp and hrc according to the terminology of Bogdanove et al. ( 13 ). hrc genes are indicated by medium gray shading. The dashed lines connect the variable-host-range and semiconserved pathogenicity regions to the central conserved region. In the host range region, the hrmBA operon of P. syringae Pss61 ( 5 , 39 ) and the avrPphE locus of P. syringae pv. phaseolicola 1302A ( 71 ) are shown to indicate the variability of the region. HrmA and AvrPphE do not show homology to each other. P. syringae pv. tomato DC3000 lacks either locus. A key to the deduced functions of the gene products is included. See Table 1 for references.
Comparison of group I and group II hrp clusters. The hrp clusters off. syringae, E. amylovora, R. solanacearum,and X. campestris are represented. Conserved hrc genes are indicated by the medium gray boxes as in Fig. 1 and are labeled above the corresponding gene. Selected hrp genes referred to in the text are labeled below the corresponding gene. The labeled black arrows in the group I hrp clusters represent conserved operons. For group I hrp clusters, note similarities in the organization of genes within individual operons. In E. amylovora,the hrp} and hrpV operons are merged into a single operon. The designations for homologs, when different, are indicated in parentheses. Only the central region of group II hrp clusters encoding apparent components of the PTC is shown. The hrp clusters of X. campestris and R. solanacearum are colinear except for the difference in the position of HrpX. X. campestris designations for selected hrp genes are shown in parentheses. Note the difference in the organization of hrc genes between the two groups of hrp clusters. See the text for references and Fig. 1 for fill pattern codes.
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