Replication regions from plant-pathogenic Pseudomonas syringae plasmids are similar to ColE2-related replicons

Many strains of the phytopathogen Pseudomonas syringae contain mutually com patible plasmids that share extensive regions of sequence homology and esse ntial replication determinants. The replication regions of two compatible l arge plasmids involved in virulence or pathogenicity, pPT23A from P. syring ae pv. tomato strain PT23 and pAV505 from P. syringae pv. phaseolicola stra in HRl1302A, were isolated. DNA sequencing of the origins of replication re vealed homologous ORFs, designated ORF-Pto and ORF-Pph, respectively. Both ORFs are 1311 bp long and encode peptides of 437 amino acids with predicted molecular masses of 48259 (Pto) and 48334 (Pph) Da. Expression of the two molecular masses of 48259 (Pto) and 48334 (Pph) Da. Expression or the two O RFs in Escherichia coli produced peptides of 50 kDa (Pto) and 56 kDa (Pph). The predicted peptides showed an overall identity of 89.7%, being highly c onserved from residues 1 to 373, but showing considerable variation in thei r C-terminal regions (50% identity over the last 64 aa). The two ORFs had s ignificant similarity with the putative replication protein from plasmid pT iK12 of Thiobacillus intermedius and other ColE2-related plasmids. However, both peptides were 100 residues longer than any of the known ColE2-related rep sequences. Subcloning of fragments from the replication region of pPT2 3A revealed the presence of at least three incompatibility determinants, de signated IncA, IncB and IncC. Partial sequencing of the region downstream o f ORF-Pto revealed homology to the rulAB genes, involved in UV resistance, from plasmid pPSR1. It is proposed that the replication origin of pPT23A se rves as the prototype of a family of related plasmids.