We are investigating a series of 6 contiguous open reading frames, (gsa, gsbA, gsbB, gsc, gsd and mpa), isolated from a low G+C content genetic element in M. avium subsp. paratuberculosis, designated GS1. Analysis of DNA and protein databases show that components of the GS element, which possess homologues in the pathogen M. tuberculosis but not M. bovis BCG, may encode functions related to extracellular polysaccharide synthesis or modification, and as such may influence the pathogenicity of M. avium subsp. paratuberculosis. We have designed constructs which facilitate the expression of the GS genes in bacterial and insect cell expression systems and the purification of the recombinant proteins utilising metal chelate chromatography. Purification of the GS components from E.coli and SF9 insect cells will provide reagents to test the diagnostic capability of the GS element. The reactivity of purified recombinant GS proteins with sera from Johne's disease cattle, Crohn's disease patients and appropriate controls can then be evaluated. In addition, GS specific anti-peptide rabbit polyclonal antibodies have been produced for use as reagents for further analysis of function of the GS element.1 A low G+C content genetic island in Mycobacterium avium subsp. paratuberculosis and M. avium subsp. silvaticum with homologous genes in M. tuberculosis.