The relapsing fever agent Borrelia hermsii has multiple copies of its chromosome and linear plasmids.
The relapsing fever agent Borrelia hermsii has multiple copies of its chromosome and linear plasmids.
复制标题
回归热病原赫氏疏螺旋体具有多个染色体拷贝和线性质粒。
DOI：
10.1093/genetics/132.2.311
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发表时间：
1992
期刊：
影响因子：
3.3
通讯作者：
Barbour,AG
中科院分区：
文献类型：
--
作者：
Kitten,T;Barbour,AG
Borrelia hermsii, a spirochete which causes relapsing fever in humans and other mammals, eludes the immune response by antigenic variation of the "Vmp" proteins. This occurs by replacement of an expressed vmp gene with a copy of a silent vmp gene. Silent and expressed vmp genes are located on separate linear plasmids. To further characterize vmp recombination, copy numbers were determined for two linear plasmids and for the 1-megabase chromosome by comparing hybridization of probes to native DNA with hybridization to recombinant plasmids containing borrelial DNA. Plasmid copy numbers were also estimated by ethidium bromide fluorescence. Total cellular DNA content was determined by spectrophotometry. For borrelias grown in mice, copy numbers and 95% confidence intervals were 14 (12-17) for an expression plasmid, 8 (7-9) for a silent plasmid, and 16 (13-18) for the chromosome. Borrelias grown in broth medium had one-fourth to one-half this number of plasmids and chromosomes. Staining of cells with 4',6-diamidino-2-phenylindole revealed DNA to be distributed throughout most of the spirochete's length. These findings indicate that borrelias organize their total cellular DNA into several complete genomes and that cells undergoing serotype switches do one or more of the following: (1) coexpress Vmps from switched and unswitched expression plasmids for at least three to five generations, (2) suppress transcription from some expression plasmid copies, or (3) partition expression plasmids nonrandomly. The lower copy number of the silent plasmid indicates that nonreciprocal Vmp gene recombination may result from loss of recombinant silent plasmids by segregation.
