Sung S Y, McDowell J V, Marconi R T
Department of Microbiology and Immunology, Medical College of Virginia at Virginia Commonwealth University, Richmond, Virginia 23298-0678, USA.
J Bacteriol. 2001 Oct;183(20):5855-61. doi: 10.1128/JB.183.20.5855-5861.2001.
In the Lyme disease spirochetes, both the ospE and vlsE gene families have been demonstrated to undergo sequence variation during infection. To further investigate the mechanisms associated with the generation of vls variation, single-nucleotide polymorphism and subsequent DNA sequence analyses were performed on the vlsE gene and its paralog, BBJ51, a related gene with a frameshift mutation. These analyses focused on a series of postinfection clonal populations obtained from mice infected with Borrelia burgdorferi B31MIpc or its clonal derivative, B31MIc53. vlsE, but not BBJ51, was found to undergo sequence changes during infection. Consistent with that reported previously (J.-R. Zhang et al., Cell 89:275-285, 1997) many of the sequence changes appear to have arisen through gene conversion events and to be localized to the variable regions of vlsE. However, analysis of the vlsE nucleotide sequences revealed that some sequence changes were the result of point mutations, as these changes did not have potential contributing sources in the vls cassettes. To determine if sequence changes accumulate in vlsE over long-term infection, the vlsE genes of clonal populations recovered after 7 months of infection in mice were analyzed. While new sequence changes developed, a significant number of these changes resulted in the restoration of the vlsE sequence of the original infecting clone. In addition, we noted that some positions within the variable regions (VR) are stable even though the cassettes possess residues that could contribute to sequence variation through gene conversion. These analyses suggest that the total number of amino acid sequence changes that can be maintained by VlsE levels off during infection. In summary, in this report we demonstrate that the development of point mutations serves as a second mechanism by which vlsE sequence variation can be generated and that the capacity for vlsE variation, while still significant, is less than previously postulated.
在莱姆病螺旋体中，ospE和vlsE基因家族在感染过程中均会发生序列变异。为进一步研究与vls变异产生相关的机制，对vlsE基因及其旁系同源基因BBJ51（一个有移码突变的相关基因）进行了单核苷酸多态性及后续DNA序列分析。这些分析聚焦于从感染伯氏疏螺旋体B31MIpc或其克隆衍生物B31MIc53的小鼠中获得的一系列感染后克隆群体。研究发现，vlsE在感染过程中会发生序列变化，而BBJ51则不会。与之前报道的结果一致（J.-R. Zhang等人，《细胞》89:275 - 285，1997），许多序列变化似乎是通过基因转换事件产生的，且定位在vlsE的可变区。然而，对vlsE核苷酸序列的分析表明，一些序列变化是点突变的结果，因为这些变化在vls盒中没有潜在的贡献来源。为确定vlsE序列变化在长期感染过程中是否会积累，对感染小鼠7个月后回收的克隆群体的vlsE基因进行了分析。虽然出现了新的序列变化，但其中相当一部分变化导致原始感染克隆的vlsE序列恢复。此外，我们注意到，尽管盒中含有可通过基因转换导致序列变异的残基，但可变区内的一些位置是稳定的。这些分析表明，感染过程中VlsE能够维持的氨基酸序列变化总数趋于平稳。总之，在本报告中，我们证明点突变的发生是vlsE序列变异产生的第二种机制，并且vlsE的变异能力虽然仍然显著，但比之前推测的要小。
