Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, MD 20742, U.S.A.
Pseudomonas syringae strains use a type III secretion system (TTSS) to translocate effector proteins that assist in the parasitism of host plant cells. Some genes for effector proteins are clustered in the exchangeable effector locus (EEL) associated with the hrp pathogenicity island. A polymerase chain reaction-based screen was developed to amplify the EEL from P. syringae strains. Of the 86 strains screened, the EEL was successfully amplified from 30 predominately North American P. syringae pv. syringae strains using hrpK and queA-derived primers and from an additional three strains using hrpL and queA-derived primers. Among the amplified EEL, ten distinct types of EEL were identified that could be classified into six families distinguishable by genetic composition, but other types of EEL may be present in strains isolated in other geographical regions. No linkage with the host range of the source strain was apparent. Gene cassettes carrying conserved flanking, coding, and intergenic sequences, present in different combinations, were identified in the characterized EEL. Six new alleles of known effectors were identified that differed from the homolog in sequence, size, or both of the gene. One of these apparently novel effector proteins, HopPsyB, retained a strongly conserved amino terminus similar to that of HopPsyA, but other regions of the two polypeptides were only weakly similar. hopPsyB was expressed from an apparent operon that included hrpK and a shcA homolog, shcB. Escherichia coli MC4100 expressing the hrp TTSS, ShcB, and HopPsyB elicited the hypersensitive response (HR) in tobacco, consistent with effector production. Indicative of translocation as an effector, P. syringae pv. tomato DC3000 expressing a HopPsyB':'AvrRpt2 fusion elicited the HR in RPS2+ Arabidopsis thaliana. P. syringae pv. tomato DC3000 carrying HopPsyB exhibited slightly enhanced virulence in several Brassica spp. These results are consistent with the hypotheses that the EEL is a source of disparate effectors functioning in pathogenicity of P. syringae strains and that it evolved independently of the hrp pathogenicity island central conserved region, most likely through integron-like assembly of transposed gene cassettes.