Unique Telomeric Expression Site of Major-Surface-Glycoprotein Genes of Pneumocystis carinii

Major cell surface glycoproteins (MSG) of Pneumocystis cariniiplay a crucial role in the host-parasite interactions involvedin P. carinii pneumonia in AIDS patients. Genes encoding MSGsare repeated, highly polymorphic, and distributed among allof the 14-15 chromosomes. Here we show, by BAL-31 exonucleasecleavage and DNA cloning experiments, that the unique expressionsite (previously termed UCS) of MSG genes located in the 500-kbchromosome is telomeric. The 11-kb genomic UCS fragment isolatedand sequenced in this study contained one MSG coding sequence(termed msg105), subtelomeric repetitive sequences and telomere-specifictandem repeats of TTAGGG oriented 5' to 3' towards the DNA end.Despite the N-terminal polymorphism, the C-terminal one-thirdsequence of MSG105 was identical to one of the known MSG-cDNAs,suggesting homologous recombination within the MSG coding sequences.These features closely resemble the Variant Surface Glycoproteinsystem of the protozoan parasite Trypanosoma brucei, suggestingthat the genetic heterogeneity of MSGs is generated by recombinationbetween the UCS expression site and multiple MSG genes by meansof reciprocal exchange or gene conversion.