Abstract: Xanthomonas oryzae pv.oryzicola (Xooc) and X.o.pv.oryzae (Xoo) are two pathovars on rice. An EcoRI-BamHI DNA fragment, corresponding to the coding region NLS and AD of AvrXa3 of Xoo, was used as a probe to screen the genomic library of Xooc. Twenty three avrBs3/PthA gene clones were obtained after positive clones of the library were digested by EcoRI endoenzyme and proved by Southern blot with the probe. Sequencing randomly selected clones, avr/Pth13 and avr/pth14, indicated that they were the members of avrBs3/PthA family of Xanthomonas pathogenic bacteria. The two putative proteins were similar as other AvrBs3/PthA, possessing the same 5′- and 3′- terminals, one leucine zipper (LZ), three nuclear localization signals (NLSs) and one acidic transcriptional activation domain (AD). However, the difference of the gene with other avrBs3/PthA family members was the repeat numbers of 34-amino-acid unit coded by 102 bp repeat. The repeat numbers of 102 bp unit was 5.5 in avr/Pth13 gene and 19.5 in avr/Pth14。Alignments of 12-13th residues in 34-amino-acid unit of Avr/Pth13 and Avr/Pth14 with those of other reported AvrBs3/PthA proteins demonstrated hypothetically that at least three HD residues in 12-13th position of 34-amino-acid unit were necessary for avirulence or/and virulence of the proteins. Pathogenicity of strain PXO99A containing avr/Pth13 and avr/Pth14 gene, respectively, on near isogenic lines (NILs) and rice cultivars with Xoo R genes showed that the virulence of PXO99A was strengthened by avr/Pth13 gene, and reduced by avr/Pth14. All the results confirmed that diverse avrBs3/PthA genes existed in Xooc as that in Xoo determining virulence in bacterial streak and blight diseases of rice. It is the first report that the avrBs3/PthA genes were cloned from Xooc and avr/Pth13 is the smallest gene in the avrBs3/PthA family up to now。

Key words: Xanthomonas oryzae pv. oryzicola, avrBs3/PthA family, Southern blot, Pathogenicity