Characterization of genomic clones encoding two microneme antigens of Sarcocystis muris (Apicomplexa).

Subgenomic libraries were constructed from Sarcocystis muris total DNA. Hybridization screening with a microneme-specific cDNA probe resulted in two clones that were sequenced. The amino acid sequences deduced showed 87% homology among each other. Three different domains were recognized within both polypeptides. Domain I includes the putative N-terminal signal sequence. Domain II represents a strongly hydrophilic region, entirely homologous in the two genes. Domain III encodes the mature polypeptides with theoretical molecular masses of 15.1 kDa each. Among 28 amino acid changes in this region, 19 replacements are conservative. The putative polypeptides carried 12 conserved cysteine residues and showed homologies with plasma kallikrein, factor XI, and an antigen of Eimeria tenella. The recombinant proteins are recognized by the monoclonal antibody 3A8 directed against the 16/17-kDa microneme antigen of S. muris cystozoites. Antiserum raised against one of the purified fusion proteins cross-reacts with its counterpart and with the native 16/17-kDa band-doublet.