IS900 targets translation initiation signals in Mycobacterium avium subsp paratuberculosis to facilitate expression of its hed gene

The Mycobacterium avium subsp. paratuberculosis (formerly Mycobacterium paratuberculosis) atypical insertion sequence, IS900, encodes a novel gene on the complementary strand to the putative transposase, p43. This gene requires a promoter, ribosome binding site (RES) and termination codon to be acquired upon insertion into the M. avium subsp. paratuberculosis genome and hence is designated the hed (host expression-dependent) gene of IS900. Analysis of IS900 insertion sites suggests that this element targets translation initiation signals in M. avium subsp. paratuberculosis, specifically inserting between the RES and start codon of a putative gene sequence. This aligns the hed initiation codon adjacent to a functional RES and possibly downstream of an active promoter, driving expression of bed protein. We have confirmed this unique targeting process by detecting expression of hed in M. avium subsp. paratuberculosis at the level of transcription by reverse transcription-PCR. Further, two bed-specific antibodies detected bed translation products in Western blots of protein extracts from M. avium subsp. paratuberculosis. A recombinant form of bed expressed and purified from Escherichia coli will facilitate studies of IS900 transposition and will also be assessed as a diagnostic antigen for M. avium subsp. paratuberculosis disease. Implications of IS900 insertion in M. avium subsp. paratuberculosis pathogenicity are discussed.