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Small RNA ArcZ regulates key virulence factors through global regulator Lrp in Erwinia amylovora

Jeffrey Schachterle: Michigan State University

<div><em>Erwinia amylovora</em> causes the devastating fire blight disease of apple and pear trees by successfully colonizing and infecting through coordinated regulation of attack and defense strategies. <em>E. amylovora </em>uses flagellar motility to migrate to susceptible host tissues, produces exopolysaccharides to build large protected populations, and translocates effectors via a type III secretion system to manipulate host cells. Each of these virulence factors is regulated by the Hfq-dependent small RNA ArcZ, and we have shown that ArcZ positively regulates flagellar motility. However, ArcZ negatively regulates translation of the flagellar master regulator <em>flhD</em> through a direct interaction. In order to clarify this contradiction and to identify additional regulators in this pathway, we conducted a transposon mutagenesis in an <em>arcZ </em>mutant background and screened 18,000 mutants for reversion to wild-type motility. This screen identified the leucine-responsive regulatory protein (Lrp) as a major regulator of flagellar motility epistatic to ArcZ. RNAseq transcriptomic analysis of the <em>arcZ </em>mutant confirmed that Lrp is a part of the ArcZ regulon. Virulence testing indicated that Lrp not only regulates flagellar motility but also production of the exopolysaccharides amylovoran and levan. These findings identify Lrp as a novel virulence regulator in <em>E. amylovora</em>, improve current understanding of flagellar regulation, and further elucidate the role of small RNAs in <em>E. amylovora </em>virulence.</div>