PATHOGENICITY, NONHOST HYPERSENSITIVITY AND HOST-DEFENSE NONPERMISSIBILITY REGULATORY GENE HRPX IS HIGHLY CONSERVED IN XANTHOMONAS PATHOVARS

Xanthomonas campestris is pv. campestris and X. oryzae pv. oryzae cont ain the 1428 base pair hrpX gene whose product is involved in the regu lation of hrp genes required for pathogenicity, non-host hypersensitiv ity and non-permissibility of compatible host defence responses. Previ ous Southern blot hybridization studies have suggested that hrpX is co nserved in several X. campestris pathovars and X. oryzae strains. We h ave confirmed and extended these findings using hrpX gene amplificatio n by polymerase chain reaction, coupled with Southern blot hybridizati on analyses. Sixteen distinct pathovars of X. campestris and 12 strain s of X. oryzae pv. oryzae were shown to contain homologs of hrpX which were not apparent in heterologous bacteria such as Agrobacterium tume faciens, A. rhizogenes, Erwinia carotovora ssp. carotovora, Pseudomona s syringae pv. glycinea, P. syringae pv. tabaci, and Escherichia coli. The hrpX gene is therefore highly conserved among Xanthomonas species and its gene product strongly resembles positive regulatory proteins of the AraC protein family.