SEQUENCE AND IMMUNOLOGICAL CHARACTERIZATION OF A SERINE-RICH ANTIGEN FROM MYCOBACTERIUM-LEPRAE

Sera from lepromatous leprosy patients were used to screen a Mycobacte rium leprae lambdagt11 library. Three positive plaques were picked, an d lysogens were constructed. Immunoblot analysis showed that all of th e lysogens expressed an apparently identical beta-galactosidase fusion protein which reacted strongly with the sera. The 1.7-kbp insert from one clone was subcloned into the lacZ gene in pUR290; sequence analys is of the end fused to lacZ revealed an open reading frame with no sig nificant homology to previously published sequences. The insert was us ed to screen an M. leprae cosmid library, and five clones were isolate d. The insert was also found to hybridize to clones expressing the M. leprae antigen which had previously been designated class III and 25L. A 1.8-kbp HindIII fragment was subcloned from one of the cosmids and sequenced. The sequence revealed a 1,227-bp open reading frame, encodi ng a 408-amino-acid protein with a predicted molecular mass of 42,466 Da. The protein contains amino- and carboxy-terminal hydrophobic domai ns and a hydrophilic central domain; the amino-terminal domain shows s ome homology to a 51-kDa hypothetical antigen of Mycobacterium tubercu losis, while the hydrophilic region contains a high proportion of seri ne residues, and we have therefore designated the protein serine-rich antigen (Sra). Some repeated motifs are present in the protein, but th eir significance is unknown. Seventy-eight percent of serum samples fr om multibacillary leprosy patients and 68% of serum samples from pauci bacillary leprosy patients recognized the fusion protein, showing that this is a major M. leprae antigen. In contrast, all serum samples fro m endemic controls were negative, while 26% of serum samples from tube rculosis patients were weakly positive.