Variant antigen genes of Trypanoso a spliced leader orphon and retenti during differentiation (life cycle/gene regulation/RNA/parasite)

Variant surface glycoprotein (VSG) gene expression in Trypanosoma brucei involves not only the se- quential activation of individual VSG genes during mammali- an bloodstream stage antigenic variation but also the regula- tion of gene expression during cyclic transmission through al- ternate mammalian and insect hosts. In the bloodstream stage, transcriptional activation of many VSG genes is correlated with the appearance of an additional copy of the gene in a novel genomic location, the expression-linked copy. The para- site loses the ability to synthesize VSG during differentiation from mammalian bloodstream to insect procyclic stage. Five different bloodstream populations were individually converted to procyclic forms. In each case, the procyclic cells retained the expression-linked copy of the bloodstream parent, and it remained in the same immediate genomic context. Transcripts homologous to the VSG structural gene exon were found in bloodstream stage RNA but not in procyclic RNA. Neverthe- less, transcripts containing sequences homologous to the VSG mRNA spliced leader were abundant in both procyclic and bloodstream stage cells. When the genomic organization of se- quences homologous to the VSG leader was examined, a spe- cific alteration correlated with procylic differentiation was found. These data are discussed in light of biological studies on antigenic variation.