Liu A Y, Van der Ploeg L H, Rijsewijk F A, Borst P
J Mol Biol. 1983 Jun 15;167(1):57-75. doi: 10.1016/s0022-2836(83)80034-5.
At the DNA level, antigenic variation in trypanosomes is brought about by the replacement of one variant surface glycoprotein (VSG) gene by another in an expression site with a strong promoter. In several cases studied, mobilization of a VSG gene for expression involves a duplication-transposition. We have determined the DNA sequence of most of the transposed segment of one such VSG, the VSG 118. At the 3' side, the transposed segment ends within the end of the gene; at the 5' side, the transposed segment is preceded by a putative VSG gene, extending our previous conclusion that VSG genes are tightly clustered. The total length of the transposed segment is about 3.5 X 10(3) base-pairs and 1.8 X 10(3) base-pairs of this codes for the VSG 118 messenger RNA. Near the 5' border of the transposed segment we find five imperfect repeats of about 70 base-pairs that are also present in front of other VSG genes, as shown by hybridization. The termini of three minor VSG 118-specific transcripts map within these repeats. The repeats have the potential to adopt non-B-DNA conformations, and could play a role in the recombination process that exchanges VSG genes in the expression site or, less likely, in pre-mRNA processing. Comparison of the DNA and the mRNA sequence has previously revealed that a terminal exon of 35 nucleotides is spliced onto the main body of the RNA. We show here that these 35 nucleotides are not in the transposed segment and they must, therefore, be contributed by the expression site. This argues persuasively that the transposition activates VSG gene expression by promoter addition rather than by a position effect.
在DNA水平上，锥虫的抗原变异是通过在具有强启动子的表达位点中，一个变异表面糖蛋白（VSG）基因被另一个所取代来实现的。在已研究的几个案例中，VSG基因的表达激活涉及重复转座。我们已经确定了其中一个这样的VSG（VSG 118）大部分转座片段的DNA序列。在3'端，转座片段在基因末端内结束；在5'端，转座片段之前是一个推定的VSG基因，这扩展了我们之前关于VSG基因紧密成簇的结论。转座片段的总长度约为3.5×10³个碱基对，其中1.8×10³个碱基对编码VSG 118信使RNA。在转座片段的5'边界附近，我们发现了大约70个碱基对的五个不完全重复序列，通过杂交表明，这些序列也存在于其他VSG基因之前。三个较小的VSG 118特异性转录本的末端定位在这些重复序列内。这些重复序列有可能形成非B-DNA构象，并可能在表达位点中交换VSG基因的重组过程中发挥作用，或者在不太可能的前体mRNA加工过程中发挥作用。DNA和mRNA序列的比较先前已揭示，一个35个核苷酸的末端外显子被剪接到RNA主体上。我们在此表明，这35个核苷酸不在转座片段中，因此它们必定由表达位点提供。这有力地表明，转座是通过添加启动子而非位置效应来激活VSG基因表达的。
