Characterization of novel coding sequences specific to Mycobacterium avium subsp. paratuberculosis: implications for diagnosis of Johne's Disease.

Mycobacterium avium subsp. paratuberculosis is the causative agent of Johne's Disease, an economically important intestinal ailment of ruminants. Due to the considerable genetic and serologic cross-reactivity with closely related and ubiquitous members of the M. avium complex, a species-specific method for the serological diagnosis of Johne's disease is unavailable. Computational and PCR-based analysis of the complete genome sequence of M. avium subsp. paratuberculosis led to the identification of 13 open reading frames with no identifiable homologs. One of these sequences is a putative insertion element present in six copies on the M. avium subsp. paratuberculosis genome. These novel M. avium subsp. paratuberculosis genes were cloned into Escherichia coli expression vectors, and nine were successfully expressed as recombinant fusion proteins. Five of these proteins were purified in sufficient amounts to allow immunoblot analyses of their reactivity with sera from naturally infected cattle as well as mice and rabbits exposed to M. avium subsp. paratuberculosis. Fusion proteins representing MAP0862, MAP3732c, and MAP2963c were recognized by nearly all of the sera tested, including those from cattle in the clinical stages of disease. Notably, further analysis of the protein encoded by MAP0862 showed that it reacted with sera from additional infected cattle but not with sera from uninfected control animals. The fusion product of MAP0860c did not react with any of the sera tested, while the remaining four proteins were variably recognized by sera from M. avium subsp. paratuberculosis-infected cattle. Collectively, the results of this study demonstrate the utility of genomic data to identify potential diagnostic sequences.