Mycobacterium paratuberculosis causes paratuberculosis, a chronic granulomatous enteritis in ruminants (Johne's disease). It is also implicated in chronic enteritis of the Crohn's disease type in humans. Milk is thought to be the transmission vector between these two diseases. M. paratuberculosis is closely related, antigenically and genetically, to M. avium. All M. paratuberculosis strains tested contain IS900, but this IS is apparently absent from M. kansasi and M. bovis BCG (Thoresen and Olsaker, 1994). IS900 is thus apparently unique to M. paratuberculosis, with about 18 copies integrated into its genome (Green et al., 1989). IS900 putative transposase (p43) is 36.6 % identical to IS110 ORF1, 21 % to IS492 ORF and 24.8 % to IS117 ORF. IS900 probably transposes through a replicative process. IS900 can be used for specific detection of M. paratuberculosis in milk (Thoresen and Olsaker, 1994) and has been developed as an integrative vector for the expression of foreign genes in mycobacteria (Dellagostin et al., 1993). In 75 Mycobacterium avium clinical isolates study, less than 25 % showed RFLP patterns with IS900 whereas 100 % gave banding patterns with IS1311 (Roiz et al., 1995). Recently, a novel gene (designated hed for host expression-dependent) has been identified on IS900: it is located on the complementary strand to the transposase, it requires a promoter, RBS and termination codon, to be acquired upon insertion into the Mycobacterium avium subsp. paratuberculosis genome. This is apparently ensured by the specificity of its insertion into sites displaying the desired features (Doran et al., 1997).
Additionnal accession number : AF416985 (Willemsen et al ; 2001; Mycobacterium avium subsp. paratuberculosis), this sequence si 1453 bp long, there is an insertion of two bp (CG) at position 36-37.