Identification of a pea pathogenicity gene cluster on a dispensable chromosome of Nectria haematococca MP VI

A cluster of genes located on the 1.6 Mb conditionally dispensable (CD) chromosome of Nectria haematococca MP VI has been cloned and identified to be the determinant of pathogenicity to pea. This gene cluster of 25 kb contains at least six genes that are expressed during the infection of the pea. In addition to PDA1, which had been identified previously, three of the genes on this cluster can independently confer some level of virulence to a isolate of Nectria haematococca lacking the CD chromosome. Based on DNA sequence analysis, PEP5 ( pea pathogenicity gene 5) is likely to encode a protein belonging to the major facilitator superfamily of proton-dependent multidrug export systems, whereas PEP2 is predicted to encode a product similar to polyadenylate binding proteins (PABPs).The PEP2 gene has been expressed in E. coli , and the PEP2 protein has been purified. Western blot analysis using polyclonal antibody against PEP2 protein and functional tests of PEP2 on pea indicate that the PEP2 protein has biological activity and is located in pea tissue adjacent to the lesions caused by infection with the fungal pathogen. The inhibitory effect of PEP2 protein on the root regeneration of detached pea seedlings is in agreement with the proposed function inferred from amino acid sequence similarity to PABPs.The deduced amino acid sequence of another transcribed portion (ORF 4) of the PEP cluster and three apparently non-transcribed open reading frames in this 25 kb region have shown a high degree of similarity to known fungal transposases. The presence of transposable elements combined with other observations such as the different G + C content of the six genes in the PEP cluster from that of genes on other Nectria haematococca chromosomes suggests an exogenous origin of this region. These features are shared by "pathogenicity islands" of pathogenic bacteria of plants and animals.