Avirulence gene avrPpiA from Pseudomonas syringae pv pisi is not required for full virulence on pea

The open reading frame encoding the avirulence gene acrPpiA1, previously cloned from race 2 of Pseudomonas syringae pv. pisi, has been subcloned and shown to express peptides corresponding to all six possible start codons. The half-life of the peptides was found to be approximately 5 min in Escherichia coli. Transposon insertions which inactivate the gene in P. syringae pv. Pisi were located both within the open reading frame and in the upstream regulatory sequence, the hrp-box. Homologues of the gene were present only in races 5 and 7, which expressed the avirulence phenotype. The gene was present on the chromosome in all race 2 isolates tested, but was plasmid-borne in the type strains of races 5 and 7. In contrast to the homologue avrRpm1 cloned from P. syringae pv. maculicola, marker-exchange mutagenesis of the gene in races 2 and 7 of P. syringae pv. pisi was not found to affect pathogenicity towards the susceptible host as indicated by symptom development or bacterial growth in the plant. (C) 1997 Academic Press Limited.