CLONING AND EXPRESSION IN ESCHERICHIA-COLI OF CDNAS ENCODING A 31-KILODALTON SURFACE-ANTIGEN OF SARCOCYSTIS-MURIS

Using polyadenylated RNA isolated from Sarcocystis muris cyst merozoites, we have constructed a cDNA library in the expression vector lambda-ZAP. Immunoscreening with monoclonal and polyclonal antibodies directed against a 31-kDa surface antigen of S. muris [1] yielded a number of clones with insert sizes ranging between 1.1 kb and 1.3 kb. An additional clone with an insert length of 1.55 kb was isolated by screening with a labeled DNA probe derived from one of the cDNA clones. The cDNA sequence was found to contain an open reading frame specifying a polypeptide of 280 amino acids with a predicted size of 29.7 kDa. The deduced amino acid sequence is rich in serine and threonine (22%) and harbors a hypothetical N-terminal signal peptide sequence as well as a C-terminal glycosyl phosphatidylinositol anchor attachment site. The predicted amino acid sequence has been confirmed by peptide sequencing and an analysis of the overall amino acid composition of the 31-kDa protein. A recombinant protein was obtained which was recognized by the polyclonal antibodies directed against the 31-kDa antigen. Antiserum raised against the purified fusion protein specifically reacted with a 31-kDa protein from S. muris cystozoites. Southern blot analysis indicated that the corresponding gene exists as a single copy within the S. muris genome.