NEC1, A GENE CONFERRING A NECROGENIC PHENOTYPE, IS CONSERVED IN PLANT-PATHOGENIC STREPTOMYCES SPP, AND LINKED TO A TRANSPOSASE PSEUDOGENE

We are investigating the genetic basis for, and evolution of, plant pa thogenicity in Streptomyces spp, The plant-pathogenic species S. scabi es, S. acidiscabies, and S, turgidiscabies cause the scab disease of p otato and produce the phytotoxins, thaxtomins. Forty-three Streptomyce s strains representing the three species were evaluated; all thaxtomin A-producing Streptomyces strains were pathogenic on potato tubers and all but one hybridized to nec1 and ORFtnp, two genes previously clone d from S, scabies ATCC 41973, nec1 confers a pathogenic phenotype on S , lividans TK24, a nonpathogen, and ORFtnp is a transposase pseudogene located 5' to nec1, The eight nonpathogenic strains tested neither pr oduced thaxtomin A nor hybridized to nec1, ORFtnp and nec1 occurred on a single PvuII restriction fragment in all thaxtomin A-producing Stre ptomyces strains, The nucleotide sequences of the homologs of nec1 and ORFtnp from two pathogenic strains each of S, scabies, S, acidiscabie s, and S, turgidiscabies were identical; oligonucleotide primers speci fic to this gene amplified homologs from all strains that hybridized t o nec1, We propose that nec1 and ORFtnp have been horizontally mobiliz ed from S, scabies to S, acidiscabies and S, turgidiscabies, and that nec1 is involved in pathogenicity and physically linked to the thaxtom in A biosynthetic genes.