In addition to virulence-related phenotypes, the presence of prophages
confers superinfection immunity to related phages. Pectobacterium atrosepticum (Pa– formerly Erwinia carotovora ssp. atroseptica) is an important potato pathogen, and due to the widespread cultivation of this food crop, Pa infections have significant BEZ235 financial implications. In common with other soft rot bacteria, the primary virulence determinants are multiple, secreted plant cell wall-degrading enzymes, although a vast array of proteins contributes to maximal pathogenicity (Corbett et al., 2005; Pemberton et al., 2005; Liu et al., 2008). Disease progression is dependent on appropriate environmental conditions. For example, anaerobic conditions inhibit oxygen-dependent host resistance mechanisms, such as phytoalexin and free radical production, as well as cell wall lignification (Perombelon, 2002). Analysis of the Pa SCRI1043 genome selleck kinase inhibitor sequence indicated the presence of 17 horizontally acquired islands (HAIs) (Bell et al., 2004). Indeed, three-quarters of the
Pa coding sequences are shared by the animal-pathogenic enterobacteria, and the plant-specific lifestyle of Pa is thought to be due in large part to the presence of these islands (Toth et al., 2006). Two of the HAIs are complete prophages (named ECA29 and ECA41 – representing HAI-9 and HAI-17, respectively), and are the subject of this study. The other HAIs impact on bacterial physiology and virulence second in multiple ways. HAI-5, for example, contains the rfb cluster, and a mutation in rfbI has been shown to result in altered lipopolysaccharide biosynthesis, reduced motility and decreased virulence (Evans et al., 2010). Mutants unable to synthesize the phytotoxin coronafacic acid (encoded on HAI-2) show markedly reduced disease on potato plants than the wild type (Bell et al., 2004). Erwinia tasmaniensis strain Et1/99
is a nonpathogenic epiphyte that is thought to compete with phytopathogenic bacteria, including other members of the Erwiniae. The 17 HAIs present in Pa are almost entirely absent from E. tasmaniensis (Kube et al., 2008). While not all virulence determinants are found on obvious HAIs (plant cell wall-degrading enzymes are not), this absence underscores the contribution of laterally transmitted genetic material to the evolution of pathogens. However, HAIs do not always play discernable roles in the virulence of phytopathogens. When two islands that encode Type III secretion systems in Erwinia amylovora were ablated, no attenuation in the ability of these strains to cause disease on pears was observed (Zhao et al., 2009). Of the 17 putative HAIs in Pa, the two prophages had not been investigated. In this study, we characterized these prophages and assessed their contribution to the pathogenicity of this economically important phytopathogen.
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