Identification of genetic and environmental factors stimulating excision from S treptomyces scabiei chromosome of the toxicogenic region responsible for pathogenicity

Summary The genes conferring pathogenicity in S treptomyces turgidiscabies , a pathogen causing common scab of potato, are grouped together on a pathogenicity island ( PAI ), which has been found to be mobile and appears to transfer and disseminate like an integrative and conjugative element ( ICE ). However, in S treptomyces scabiei , another common scab‐inducing species, the pathogenicity genes are clustered in two regions: the toxicogenic region ( TR ) and the colonization region. The S . scabiei 87.22 genome was analysed to investigate the potential mobility of the TR . Attachment sites ( att ), short homologous sequences that delineate ICEs , were identified at both extremities of the TR . An internal att site was also found, suggesting that the TR has a composite structure ( TR 1 and TR 2). Thaxtomin biosynthetic genes, essential for pathogenicity, were found in TR 1, whereas candidate genes with known functions in recombination, replication and conjugal transfer were found in TR 2. Excision of the TR 1 or TR 2 subregions alone, or of the entire TR region, was observed, although the excision frequency of TR was low. However, the excision frequency was considerably increased in the presence of either mitomycin C or S treptomyces coelicolor cells. A composite TR structure was not observed in all S . scabiei and S treptomyces acidiscabies strains tested. Of the ten strains analysed, seven lacked TR 2 and no TR excision event could be detected in these strains, thus suggesting the implication of TR 2 in the mobilization of S . scabiei   TR .