Polyploidy, regular patterning of genome copies, and unusual control of DNA partitioning in the Lyme disease spirochete

Borrelia burgdorferi, the tick-transmitted spirochete agent of Lyme disease, has a highly segmented genome with a linear chromosome and various linear or circular plasmids. Here, by imaging several chromosomal loci and 16 distinct plasmids, we show that B. burgdorferi is polyploid during growth in culture and that the number of genome copies decreases during stationary phase. B. burgdorferi is also polyploid inside fed ticks and chromosome copies are regularly spaced along the spirochete's length in both growing cultures and ticks. This patterning involves the conserved DNA partitioning protein ParA whose localization is controlled by a potentially phage-derived protein, ParZ, instead of its usual partner ParB. ParZ binds its owe coding region and acts as a centromere-binding protein. While ParA works with ParZ, ParB controls the localization of the condensin, SMC. Together, the ParA/ParZ and ParB/SMC pairs ensure faithful chromosome inheritance. Our findings underscore the plasticity of cellular functions, even those as fundamental as chromosome segregation. Overall design: ChIP-seq experiments were performed on wild type and mutant cells of Borrelia burgdorferi S9 or M31 growing in BSK-II. WGS files were used as input control samples for ChIP-seq experiments.

伯氏疏螺旋体（Borrelia burgdorferi）是经蜱虫传播的莱姆病（Lyme disease）螺旋体病原体，其基因组具有高度分段特性，包含一条线性染色体与多种线性或环状质粒。本研究通过对多个染色体位点以及16种不同质粒进行成像分析，证实伯氏疏螺旋体在体外培养增殖过程中处于多倍体状态，且基因组拷贝数在稳定期会逐渐降低。此外，伯氏疏螺旋体在饱血蜱虫体内同样为多倍体；无论在培养体系还是蜱虫体内，染色体拷贝均沿螺旋体体长呈规律性均匀排布。该排布模式依赖于保守的DNA分离蛋白ParA，其定位由潜在噬菌体来源的蛋白ParZ调控，而非其常规伴侣蛋白ParB。ParZ可结合自身编码区域，并充当着丝粒结合蛋白的角色。当ParA与ParZ协同发挥功能时，ParB则调控凝缩蛋白SMC的定位。综上，ParA/ParZ与ParB/SMC这两组蛋白对共同确保了染色体的忠实遗传。本研究结果凸显了细胞功能的可塑性，即便诸如染色体分离这类极为基础的细胞过程亦是如此。实验设计概述：对在BSK-II培养基中培养的伯氏疏螺旋体S9或M31的野生型与突变体细胞开展染色质免疫沉淀测序（ChIP-seq，Chromatin Immunoprecipitation Sequencing）实验，全基因组测序（WGS，Whole Genome Sequencing）文件被用作ChIP-seq实验的输入对照样本。