MSG gene cluster encoding major cell surface glycoproteins of rat Pneumocystis carinii.

The MSG genes of Pneumocystis carinii encode major cell surface glycoproteins of multi-gene families and play crucial roles in the pathobiology of P. carinii pneumocystosis. The 11,370-bp chromosomal DNA encoding MSG was cloned and sequenced. It contained three open reading frames in tandem repeat, MSG99, MSG100 and ORF-3. MSG99 and MSG100 shared 39% protein sequence identity and belonged to the same MSG family, regardless of the internal ca. 1,000-bp deletion in MSG99. ORF-3 encodes a putative 47,820-Da protein and had no sequence similarities to MSG molecules or any known proteins. However, the ORF-3 protein was rich in proline residues and highly hydrophilic except for the C-terminal region, which seemed to be an anchoring sequence in the cell membrane. Northern and Southern blot hybridization analyses revealed a 3-kb ORF-3 transcript as well as multiple DNA fragments carrying the MSG and ORF-3 sequences. These results strongly suggested that diverse MSG genes are clustered in tandem repeats and these clusters involve ORF-3. The possible association of ORF-3 protein in the cell membrane is discussed.