Liu A Y, Michels P A, Bernards A, Borst P
J Mol Biol. 1985 Apr 5;182(3):383-96. doi: 10.1016/0022-2836(85)90198-6.
We have studied further the genes for trypanosomal variant surface glycoproteins expressed during a chronic infection of rabbits with Trypanosoma brucei, strain 427. We show that there are three closely related chromosomal-internal isogenes for VSG 121; expression of one of these genes is accompanied by the duplicate transposition of the gene to a telomeric expression site, also used by other chromosome-internal VSG genes. The 3' end of the 121 gene is replaced during transposition with another sequence, also found in the VSG mRNAs of two other variants. We infer that an incoming VSG gene duplicate recombines with the resident gene in the expression site and may exchange ends in this process. The extra expression-linked copy of the 121 gene is lost when another gene enters the expression site. However, when the telomeric VSG gene 221 is activated without duplication the extra 121 gene copy is inactivated without detectable alterations in or around the gene. We have also analysed the VSG genes expressed very early when trypanosomes are introduced into rats or tissue culture. The five genes identified in 24 independent switching events were all found to be telomeric genes and we calculate that the telomeric 1.8 gene has a 50% chance of being activated in this trypanosome strain when the trypanosome switches the VSG that is synthesized. We argue that the preferential expression of telomeric VSG genes is due to two factors: first, some telomeric genes reside in an inactive expression site, that can be reactivated; second, telomeric genes can enter an active expression site by a duplicative telomere conversion and this process occurs more frequently than the duplicative transposition of chromosome-internal genes to an expression site.
我们进一步研究了布氏锥虫427株慢性感染家兔期间表达的锥虫可变表面糖蛋白（VSG）基因。我们发现VSG 121有三个紧密相关的染色体内部同基因；其中一个基因的表达伴随着该基因重复转座到一个端粒表达位点，其他染色体内部VSG基因也使用这个位点。121基因的3'端在转座过程中被另一个序列取代，该序列也存在于另外两个变体的VSG mRNA中。我们推断，进入的VSG基因重复序列与表达位点中的常驻基因重组，并可能在此过程中交换末端。当另一个基因进入表达位点时，121基因的额外表达相关拷贝会丢失。然而，当端粒VSG基因221在没有重复的情况下被激活时，额外的121基因拷贝会失活，而该基因内部或周围没有可检测到的改变。我们还分析了锥虫引入大鼠或组织培养后早期表达的VSG基因。在24次独立的转换事件中鉴定出的5个基因均为端粒基因，我们计算出，当锥虫转换合成VSG时，端粒1.8基因在该锥虫菌株中有50%的机会被激活。我们认为端粒VSG基因的优先表达归因于两个因素：第一，一些端粒基因位于一个可重新激活的非活性表达位点；第二，端粒基因可以通过复制性端粒转换进入一个活性表达位点，并且这个过程比染色体内部基因向表达位点的复制性转座更频繁地发生。
