A XANTHOMONADIN-ENCODING GENE-CLUSTER FOR THE IDENTIFICATION OF PATHOVARS OF XANTHOMONAS-CAMPESTRIS

A gene cluster for xanthomonadin production was obtained from Xanthomonas campestris pv. campestris, the causal agent of black rot of crucifers. Eighteen mutants affected in pigment production were identified. Seventeen of these mutants were fully pathogenic, and they could be divided into four classes based on the absorption spectra of pigment extracts. Sequences from a 25.4-kbp genomic clone (pIG102) restored pigment production to all 18 mutants and to a naturally occurring nonpigmented strain of X. c. pv. magniferae-indicae. Analysis of pIG102 by subcloning and mutant restoration identified six functional domains for pigment production, and this genomic region conferred the production of a xanthomonadin pigment to Pseudomonas fluorescens. Clone pIG102 and subcloned derivatives encoding various functional domains were used as DNA probes in colony blot hybridizations with genomic DNA of 105 strains of Xanthomonas, Pseudomonas, Erwinia, and Clavibacter. Clone pIG102 hybridized strongly to all strains of 18 pathovars of X. campestris and showed little or no hybridization to the other strains. In Southern hybridizations, there was no strict correlation between pathovar designation and the restriction fragment length polymorphism pattern of the pigment-encoding region. A pIG102 subclone, pIG233, hybridized to all of the X. campestris strains but showed no hybridization to the non-xanthomonads. These probes should be useful in distinguishing X. campestris pathovars from other genera of bacteria associated with plants.