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Xanthomonas oryzae pv. oryzae ( Xoo) is the causal agent of bacterial leaf blight, a serious disease of rice. Here, we isolated a pathogenicity-deficient mutant of Xoo strain T7174R [a rifampicin-resistant derivative of T7174 (MAFF311018)] produced by in vivo insertion mutagenesis using the EZ::TN™ transposon system. This mutant, 74M2, was significantly reduced in virulence on rice cultivar IR24. From sequence analyses, the region with the EZ::TN transposon insertion was found to be in ORF XOO1662, one of the leucine-rich proteins found in the genome database for MAFF311018, which we have named lrpX. In planta growth of 74M2 was also slower than that of the wild type. A complementation construct for 74M2 restored both virulence and in planta growth to wild-type levels. The promoter region of lrpX contained a consensus plant-inducible promoter (PIP)-box (i.e., TTCGC-N15-TTCGC) commonly found in HrpX (an AraC-type transcriptional regulator)-regulon in xanthomonads. The HrpXo (HrpX of Xoo)-dependent expression of lrpX was confirmed using the fusion construct with lrpX:: GUS in hrp-inducing minimal medium (XOM2). The expression of lrpX was predicted to be self-regulated by its own translation product by directly binding to its own promoter. [ABSTRACT FROM AUTHOR]