ANTIGENIC VARIATION BY POSITIONAL CONTROL OF MAJOR SURFACE GLYCOPROTEIN GENE-EXPRESSION IN PNEUMOCYSTIS-CARINII

Major cell surface glycoproteins (MSGs) of Pneumocystis carinii play a crucial role in host-parasite interactions during P. carinii pneumocy stosis in AIDS patients, Genes encoding MSGs are repeated and disperse d throughout the genome and are highly polymorphic, MSG gene expressio n was found to be mediated by a DNA element that was termed the upstre am conserved sequence (UCS). The UCS element maps to a single chromoso me, is attached to expressed MSG genes, and encodes the sequence found at the 5' ends of most MSG mRNAs. The UCS is not highly repeated, but P. carinii populations contain many different MSG genes attached to t he UCS, suggesting that different organisms in the population have dif ferent MSG genes attached to the UCS, Such genetic heterogeneity may b e generated by recombination between MSG genes at the UCS locus and on e or more of the dozens of MSG genes located elsewhere in the genome.