Characterization of an insertion sequence element associated with genetically diverse plant pathogenic Streptomyces spp.

Streptomycetes are common soil inhabitants, yet few described species are p lant pathogens. While the pathogenicity mechanisms remain unclear, previous work identified a gene, neel, which encodes a putative pathogenicity or vi rulence factor. nec1 and a neighboring transposase pseudogene, ORFtnp, are conserved among unrelated plant pathogens and absent from nonpathogens. The atypical GC content of nec1 suggests that it was acquired through horizont al transfer events. Our investigation of the genetic organization of region s adjacent to the 3' end of nec1 in Streptomyces scabies 84.34 identified a new insertion sequence (IS) element, IS1629, with homology to other IS ele ments from prokaryotic animal pathogens. IS1629 is 1,462 bp with 26-bp term inal inverted repeats and encodes a putative 431-amino-acid (aa) transposas e. Transposition of IS1629 generates a 10-bp target site duplication. A 77- nucleotide (nt) sequence encompassing the start codon and upstream region o f the transposase was identified which could function in the posttranscritp ional regulation of transposase synthesis. A functional copy of IS1629 from S. turgidiscabies 94.09 (Hi-C-13) was selected in the transposon trap pCZA 126, through its insertion into the lambda cI857 repressor. IS1629 is prese nt in multiple copies in some S. scabies strains and is present in all S. a cidiscabies and S. turgidiscabies strains examined. A second copy of IS1629 was identified between ORFtnp and nec1 in S. acidiscabies strains. The div ersity of IS1629 hybridization profiles was greatest within S. scabies. IS1 629 was absent from the 27 nonpathogenic Streptomyces strains tested. The g enetic organization and nucleotide sequence of the nec1-IS1629 region was c onserved and identical among representatives of S. acidiscabies and S. turg idiscabies. These findings support our current model for the unidirectional transfer of the ORFtnp-nec1-IS1629 locus from IS1629-containing S. scabies (type II) to S. acidiscabies and S. turgidiscabies.