Phylogeny of the replication regions of pPT23A-like plasmids from Pseudomonas syringae

It was previously shown that most Pseudomonas syringae strains contain one or more plasmids with cross-hybridizing replication regions and other areas of homology, and these plasmids were designated the pPT23A-like family. Th e majority of these plasmids encode genes conferring epiphytic fitness or r esistance to antibacterial compounds and those investigated in this study a re essential for pathogenicity or increased virulence. The phylogeny of 14 pPT23A-like plasmids from five P. syringae pathovars was studied by compari ng a fragment of the sequence of their repA genes (encoding a replicase ess ential for replication). In the phylogenetic tree obtained, four groups (gr eater than or equal to 88.8 % identity between their members) could be iden tified. The first group contained the plasmids from three P. syringae pv. t omato strains, a P. syringae pv. apii strain and five out of the seven P. s yringae pv. syringae strains, with identity ranging between 88.8 and 100%. The clustering of the pv. syringae strains did not reflect host specializat ion or previously reported phylogenetic relationships. The second group con tained the plasmids from two strains of pv. glycinea and pv. tomato (95.5% identity), and it also included the previously sequenced replicon of a path ogenicity plasmid from P. syringae pv. phaseolicola. The plasmids from the remaining two pv. syringae strains were distantly related to the other plas mid sequences. Hybridization experiments using different genes or transposa ble elements previously described as plasmid-borne in P. syringae, showed t hat the gene content of highly related plasmids could be dissimilar, sugges ting the occurrence of major plasmid reorganizations. Additionally, the phy logeny of the different native plasmids did not always correlate with the p hylogeny of their harbouring strains, as determined by the analysis of extr agenic repetitive consensus (ERIC) and arbitrarily primed PCR (AP-PCR) prod ucts. Collectively, these results suggest that pPT23A-like plasmids were, i n most cases, acquired early during evolution.