Expression of the Pneumocystis carinii major surface glycoprotein epitope is correlated with linkage of the cognate gene to the upstream conserved sequence locus.

The major surface glycoprotein (MSG) is a variable surface antigen of the pathogenic fungus Pneumocystis carinii. Many forms of MSG are encoded by a gene family. Expression of the MSG gene family is believed to be controlled in a cis-dependent fashion. Transcription of a given MSG gene is correlated with linkage of that gene to a unique locus called the upstream conserved sequence (UCS). These data predict that the MSG protein on a given organism will match that encoded by the MSG gene at the UCS locus in that organism. To test this hypothesis, a monoclonal antibody (mAb) that recognizes a small number of MSG isoforms was identified, and the DNA sequence encoding the mAb epitope (epitope-encoding sequence, EES) was determined. Western blotting, immunofluorescence and DNA hybridization showed that expression of the mAb epitope was associated with the presence of the EES at the UCS locus. Correlation of epitope expression and UCS linkage supports the hypothesis that expression of a particular MSG on the surface requires UCS linkage of the gene encoding it.