Rothwell V, Aline R, Parsons M, Agabian N, Stuart K
Nature. 1985;313(6003):595-7. doi: 10.1038/313595a0.
African trypanosomes contain numerous variant surface glycoprotein (VSG) genes, but express only one at a time. When different VSG genes are activated by gene duplicative or non-duplicative mechanisms, antigenic variation occurs. Although transcriptionally inactive VSG genes can have either an intra-chromosomal or a telomeric location, all expressed VSG genes so far examined are telomeric. Some VSG genes are located on minichromosomes of approximately 100 kilobases (kb), but all those thus far described are transcriptionally inactive. We have found that the single copy of the IsTat 1.1 VSG gene in the IsTaR 1 serodeme has a telomeric location on a minichromosome when inactive. When it is activated, it is not duplicated and remains on a minichromosome. These data indicate that minichromosomes contain or can acquire all the DNA sequences necessary in cis for VSG gene expression and antigenic switching. Of several sequences associated with expressed VSG genes or their transcripts, only the 76-base pair (bp) repeat element was found in minichromosomal DNA. Variant antigenic types (VATs) which occurred immediately before and after VAT-1, expressed VSG genes located on larger chromosomes. Thus, different chromosomes can act as VSG gene expression sites.
非洲锥虫含有众多的变异表面糖蛋白（VSG）基因，但一次仅表达其中一个。当不同的VSG基因通过基因复制或非复制机制被激活时，就会发生抗原变异。尽管转录不活跃的VSG基因可位于染色体内部或端粒位置，但迄今为止所检测的所有表达的VSG基因均位于端粒。一些VSG基因位于约100千碱基（kb）的微型染色体上，但迄今为止所描述的所有这些基因均转录不活跃。我们发现，在IsTaR 1血清型中，IsTat 1.1 VSG基因的单拷贝在不活跃时位于微型染色体的端粒位置。当它被激活时，不会发生复制，而是保留在微型染色体上。这些数据表明，微型染色体包含或能够获得VSG基因表达和抗原转换所需的所有顺式DNA序列。在与表达的VSG基因或其转录本相关的几个序列中，仅在微型染色体DNA中发现了76碱基对（bp）的重复元件。紧接在VAT-1之前和之后出现的变异抗原类型（VAT），表达的VSG基因位于较大的染色体上。因此，不同的染色体可作为VSG基因的表达位点。
