Sequence diversity of rulA among natural isolates of Pseudomonas syringae and effect on function of rulAB-mediated UV radiation tolerance

The rulAB locus confers tolerance to UV radiation and is borne on plasmids of the pPT23A family in Pseudomonas syringae, We sequenced 14 rulA alleles from P. syringae strains representing seven pathovars and found sequence di fferences of 1 to 12% within pathovar syringae, and up to 15% differences b etween pathovars. Since the sequence variation within rulA was similar to t hat of P, syringae chromosomal alleles, we hypothesized that rulAB has evol ved over a long time period in P. syringae, A phylogenetic analysis of the deduced amino acid sequences of rulA resulted in seven clusters. Strains fr om the same plant host grouped together in three cases; however, strains fr om different pathovars grouped together in two cases. In particular, the ru lA alleles from P. syringae pv. lachrymans and P. syringae pv, pisi were gr ouped but were clearly distinct from the other sequenced alleles, suggestin g the possibility of a recent interpathovar transfer. We constructed chimer ic rulAB expression clones and found that the observed sequence differences resulted in significant differences in UV (wavelength) radiation sensitivi ty. Our results suggest that specific amino acid changes in RulA could alte r UV radiation tolerance and the competitiveness of the P. syringae host in the phyllosphere.