Erwinia amylovora is a Gram-negative bacterium and the causal agent of fire blight, a destructive disease of rosaceous species such as apple and pear. The genetic determinants of fire blight disease development include a diverse array of virulence mechanisms including biofilm formation and the type III secretion system (T3SS). The activity of these virulence factors in E. amylovora is coordinated by a complex system of regulatory elements that interact with all stages of gene expression and function. As such, the characterization of virulence regulators is a powerful tool for understanding host-microbe interactions via the identification of downstream virulence mechanisms. In E. amylovora, multiple virulence regulators control disease development including the alternative sigma factor HrpL. HrpL is conserved in many bacterial plant pathogens and plays a vital role the transcription of both structural and translocated components of the T3SS. While the role of HrpL in regulating the T3SS is indispensible for pathogenesis, the HrpL regulon also includes a large number of uncharacterized genes that do not have predicted roles in type III secretion. Because the HrpL regulon is a quintessential virulence regulon, uncharacterized genes regulated by HrpL are likely to play important roles in disease development. Using a combined approach including microarray expression analyses, bioinformatics, and mutagenesis, six novel virulence factors were identified including a biofilm regulator NlpI (EAM_3066), a transcriptional regulator YdcN (EAM_1248) and a HrpL-regulated gene cluster including EAM_2938. -- Abstract.
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