Excision from tRNA genes of a large chromosomal region, carrying avrPphB, associated with race change in the bean pathogen, Pseudomonas syringae pv. phaseolicola

Pseudomonas syringae pv. phaseolicola (Pph) race 4 strain 1302A carries avi rulence gene avrPphB. Strain RJ3, a sectoral variant from a 1302A culture, exhibited an extended host range in cultivars of bean and soybean resulting from the absence of avrPphB from the RJ3 chromosome. Complementation of RJ 3 with avrPphB restored the race 4 phenotype. Both strains showed similar i n planta growth in susceptible bean cultivars. Analysis of RJ3 indicated lo ss of > 40 kb of DNA surrounding avrPphB. Collinearity of the two genomes w as determined for the left and right junctions of the deleted avrPphB regio n; the left junction is approximate to 19 kb and the right junction > 20 kb from avrPphB in 1302A. Sequencing revealed that the region containing avrP phB was inserted into a tRNA(LYS) gene, which was re-formed at the right ju nction in strain 1302A. A putative lysine tRNA pseudogene (Psi tRNA(LYS)) w as found at the left junction of the insertion. All tRNA genes were in iden tical orientation in the chromosome. Genes near the left junction exhibited predicted protein homologies with gene products associated with a virulenc e locus of the periodontal pathogen Actinobacillus actinomycetemcomitans. S pecific oligonucleotide primers that differentiate 1302A from RJ3 were desi gned and used to demonstrate that avrPphB was located in different regions of the chromosome in other strains of Pph. Deletion of a large region of th e chromosome containing an avirulence gene represents a new route to race c hange in Pph.