Parsons M, Nelson R G, Stuart K, Agabian N
Proc Natl Acad Sci U S A. 1984 Feb;81(3):684-8. doi: 10.1073/pnas.81.3.684.
Variant surface glycoprotein (VSG) gene expression in Trypanosoma brucei involves not only the sequential activation of individual VSG genes during mammalian bloodstream stage antigenic variation but also the regulation of gene expression during cyclic transmission through alternate 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 parasite 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. Nevertheless, transcripts containing sequences homologous to the VSG mRNA spliced leader were abundant in both procyclic and bloodstream stage cells. When the genomic organization of sequences homologous to the VSG leader was examined, a specific alteration correlated with procylic differentiation was found. These data are discussed in light of biological studies on antigenic variation.
布氏锥虫中的变异表面糖蛋白（VSG）基因表达不仅涉及哺乳动物血流阶段抗原变异期间单个VSG基因的顺序激活，还涉及通过交替的哺乳动物和昆虫宿主进行循环传播期间的基因表达调控。在血流阶段，许多VSG基因的转录激活与该基因在一个新的基因组位置（表达关联拷贝）出现额外拷贝相关。寄生虫在从哺乳动物血流阶段分化为昆虫前循环阶段时失去合成VSG的能力。五个不同的血流群体分别被转化为前循环形式。在每种情况下，前循环细胞保留了血流亲本的表达关联拷贝，并且它仍处于相同的直接基因组环境中。与VSG结构基因外显子同源的转录本在血流阶段RNA中被发现，但在前循环RNA中未被发现。然而，含有与VSG mRNA剪接前导序列同源序列的转录本在循环前和血流阶段细胞中都很丰富。当检查与VSG前导序列同源的序列的基因组组织时，发现了与前循环分化相关的特定改变。根据关于抗原变异的生物学研究对这些数据进行了讨论。
