A dispensable region of the chromosome which is associated with an avirulence gene in Pseudomonas syringae pv. pisi

Pseudomonas syringae pv. pisi comprises a number of races which fall into t wo phylogenetically distinct groups (designated I and II). Races are based on cultivar specificity in the host plant, pea (Pisum sativum), and are spe cified by the presence of avirulence genes. The avirulence gene avrPpiA1 is present on the chromosome of all strains examined in race 2, which belongs to phylogenetic group II. A race 4B strain, from phylogenetic group I, lac ks this avirulence gene and a comparative study was made of the chromosome in strains representing these two races. A race 2 cosmid clone (pAV270) car rying avrPpiA1 was used as a basis for collinearity analysis of races 2 and 4B. A region of the chromosome amounting to 8.5 kb and including avrPpiA1 was absent from race 4B compared with race 2. A fragment spanning the junct ion of the discontinuity in race 4B was isolated, cloned and used to delimi t the extent of the additional DNA present in race 2. In both races the bor ders of the discontinuity contained DNA sequences which showed a high degre e of conservation. A 7 bp slightly imperfect direct repeat (CCAGCT/AT) flan ked the additional DNA in race 2, with a single copy in race 4B. The region flanking the additional DNA was present in all races of P. syringae pv, pi si. These results confirm the phylogenetic groupings in P. syringae pv. pis i.