Evidence for the mobility of an avirulence gene, avrPpiA1, between the chromosome and plasmids of races of Pseudomonas syringae pv. pisi

An 8.5-kb DNA fragment containing the avirulence gene avrPpiA1 is present in the chromosome of race 2 but not present in race 4B of Pseudomonas syringae pv. pisi. The likely acquisition of avrPpiA1 in race 2 at some time in the past was indicated by the presence of direct repeat sequences at the boundaries of the 8.5-kb region, which was amplified by polymerase chain reaction (PCR), cloned and sequenced. The sequence analysis of the 8.5-kb fragment revealed the presence of rulAB genes, which are usually associated with plasmids. The rulB gene had been disrupted by a 4.3-kb length of DNA, which included avrPpiA1 and open reading frames (ORFs) with similarity to bacteriophage and transposase genes. Inverted repeats flanked the 4.3-kb region reinforcing the possibility that a further insertion had occurred in this region. The results presented here provide evidence for the horizontal transfer of an avirulence gene and suggest that this region may be a hotspot for recombination with the chromosome.