Kitten T, Barbour A G
Department of Microbiology, University of Texas Health Science Center, San Antonio 78284.
Proc Natl Acad Sci U S A. 1990 Aug;87(16):6077-81. doi: 10.1073/pnas.87.16.6077.
Borrelia hermsii, an agent of relapsing fever, survives in mammals through antigenic variation. Change in serotype-specific variable outer membrane proteins (Vmps) occurs when a Vmp gene at an expression site is replaced with a previously silent gene for another Vmp. Silent and active genes are on separate linear plasmids. The upstream site for a nonreciprocal recombination between two linear plasmids is near the 5' ends of the expressed and silent genes. In the present study we sought the downstream recombination sites in two serotypes, 7 and 21. Restriction fragments containing plasmid telomeres were identified by susceptibility to digestion with BAL-31 and rapid reannealment following denaturation. Whereas both silent genes and a minority population of both expression-linked genes were several kilobases from the telomeres, the predominant population of both expressed genes had 3' ends near plasmid telomeres. Sequence analysis of the predominant expression plasmids revealed that the telomeric sequences were the same in serotypes 7 and 21. Identical sequence was also downstream of silent Vmp genes. Switching of Vmp genes appears to occur by recombination that involves both upstream and downstream sites. The expression plasmid's telomere is preserved in the recombination event.
赫氏疏螺旋体是回归热的病原体，它通过抗原变异在哺乳动物体内存活。当表达位点的一个Vmp基因被另一个之前沉默的Vmp基因取代时，血清型特异性可变外膜蛋白（Vmps）就会发生变化。沉默基因和活性基因位于不同的线性质粒上。两个线性质粒之间非相互重组的上游位点靠近表达基因和沉默基因的5'端。在本研究中，我们寻找了血清型7和21中的下游重组位点。通过对BAL-31消化的敏感性以及变性后快速复性来鉴定含有质粒端粒的限制性片段。沉默基因以及与表达相关的基因的少数群体距离端粒有几千个碱基对，而两个表达基因的主要群体的3'端靠近质粒端粒。对主要表达质粒的序列分析表明，血清型7和21中的端粒序列相同。沉默Vmp基因的下游也有相同的序列。Vmp基因的转换似乎是通过涉及上游和下游位点的重组发生的。表达质粒的端粒在重组事件中得以保留。
