Type-II Major-Surface-Glycoprotein Family of Pneumocystis carinii under the Control of Novel Expression Elements

Pneumocystis carinii are coated by abundant surface proteins, named MSGs for major surface glycoproteins, that undergo antigenic variation. There are many MSG genes on the chromosomes but the MSG expression site is restricted to a unique site called type-I UCS, and silent MSG genes can be translocated to the expression site by DNA recombination. We have reported previously the existence of alternative UCS elements named type-II UCS by MSG transcript analyses (J. Infect. Dis., 171, 1563-1568 [1995]). In this study we characterized the features of type-II UCS and MSG genes that are under the control of type-II UCS (hence designated the type-II MSG family) by cDNA and genomic analyses. Contrary to type-I UCS, type-II UCS elements are located at most of the P. carinii chromosomes and are directly attached to type-II MSG genes. Type-II MSG genes consist of two subtypes in coding sizes of 2.3 kb and 3.3 kb. Antibody raised against the synthetic type-II UCS polypeptide detected products in molecular masses ranging from 90 to 100 kDa. Although the cloned type-II MSG sequences are significantly diverse, cysteine residues that are rich and highly conservative in type-I MSGs are also strictly conserved in type-II MSG proteins, showing the importance of multiple cysteines in the structural integrity of MSGs in the cell surface of P. carinii.