A low G+C content genetic island in Mycobacterium avium subsp. paratuberculosis and M-avium subsp. silvaticum with homologous genes in Mycobacterium tuberculosis

The technique of representation difference analysis PCR has been applied to find genes specific to Mycobacterium avium subsp. paratuberculosis. This g enerated a 671 bp fragment which was used to isolate a larger genetic eleme nt found in the enteric pathogens M. avium subsp. paratuberculosis and M. a vium subsp. silvaticum but which was absent from the very closely related a nd relatively benign M. avium subsp. avium. This element, designated GS, is greater than 6.5 kbp in length and has a G+C content 9 mol% lower than oth er genes from this species. There is a previously uncharacterized insertion sequence associated with one end. The GS element encodes five ORFs in M. a vium subsp. paratuberculosis and M. avium subsp. silvaticum, all of which h ave counterparts encoded in Mycobacterium tuberculosis. Database searches r evealed homologues for these ORFs in a number of bacterial species, predomi nantly Gram-negative organisms, including a number of enteric pathogens. Th ese homologous genes encode functions related to LPS or extracellular polys accharide biosynthesis. This element has a number of features in common wit h pathogenicity islands such as its low G+C content, an association with a putative insertion sequence and a grouping of genes of related function wit h a possible link to virulence. No direct link to pathogenicity has been sh own but GS may belong to a group of related 'genetic islands' and represent s the first such element to be identified in mycobacteria.