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The mechanism of xylose-dependent expression of hrp genes in a rice pathogen Xanthomonas oryzae pv. oryzae

Yumi Ikawa: Kyoto Prefectural University

<div><em>hrp </em>genes encoding components of the type III secretion system are essential for virulence of<em> Xanthomonas oryzae</em> pv. <em>oryzae</em> (Xoo), the bacterial pathogen of leaf blight of rice. Expression of<em> hrp</em> genes is regulated by two key regulators, HrpG and HrpX. Previously, we reported the importance of xylose in <em>hrp</em> gene expression in the bacterium, and that, without xylose, the accumulation of a <em>hrp</em> regulator, HrpX, not the expression of the protein, is decreased. Here, to clarify the mechanism of quantitative regulation of HrpX, we conducted random transposon mutagenesis, and found that a mutant with a transposon in a LacI-type transcriptional regulator XylR showed high <em>hrp</em> gene expression even under the xylose-free medium. The<em> xylR</em> deletion mutant showed high HrpX accumulation and <em>hrp</em> gene expression in the medium without xylose, similarly to the wild type incubated in the xylose-containing medium. XylR is known to negatively regulate xylan/xylose metabolism-related genes, including a xylose isomerase gene <em>xylA2</em>, in other bacteria. We confirmed the involvement of Xoo XylR in the negative regulation of <em>xylA2</em> and, furthermore, the binding of the protein to the upstream region of the <em>xylA2</em>. These results suggest that, in the presence of xylose, inactivation of XylR activates xylan/xylose utilization and, simultaneously, of <em>hrp</em> gene expression through the higher accumulation of HrpX in Xoo. The mechanism of the XylR-dependent quantitative regulation of HrpX will be discussed.</div>