NOVEL EXCHANGEABLE EFFECTOR LOCI ASSOCIATED WITH THE PSEUDOMONAS SYRINGAE HRP PATHOGENICITY ISLAND: EVIDENCE FOR INTEGRON-LIKE ASSEMBLY FROM TRANSPOSED GENE CASSETTES

Abstract

<i>Pseudomonas syringae</i> strains use a type III secretion system (TTSS) to translocate effector proteins that assist in the parasitism of host plant cells. Some genes encoding effector proteins are clustered in the exchangeable effector locus (EEL) associated with the <i>hrp</i> pathogenicity island. A PCR-based screen was developed to amplify the EEL from <i>P. syringae</i> strains. Of the 86 strains screened, the EEL was successfully amplified from 29 predominately North American <i>P. syringae</i> pv. syringae strains using <i>hrpK</i> and <i>queA</i>-derived primers. Among the amplified EEL, ten distinct types of EEL were identified that could be classified into six families distinguishable by genetic composition. These results are consistent with the hypotheses that the EEL is a source of disparate effectors involved with the pathogenicity of <i>P. syringae</i> strains and that the EEL evolved independently of the central conserved region of the <i>hrp</i> pathogenicity island, possibly by integron-like assembly of transposed gene cassettes.