Fischer G, Wenner T, Decaris B, Leblond P
Laboratoire de Génétique et Microbiologie, Unité associée INRA 952, Université Henri Poincaré-Nancy 1, Faculté des Sciences, Boulevard des Aiguillettes, F-54506 Vandoeuvre-lès-Nancy, France.
Proc Natl Acad Sci U S A. 1998 Nov 24;95(24):14296-301. doi: 10.1073/pnas.95.24.14296.
The chromosomal DNA of the bacteria Streptomyces ambofaciens DSM40697 is an 8-Mb linear molecule that ends in terminal inverted repeats (TIRs) of 210 kb. The sequences of the TIRs are highly variable between the different linear replicons of Streptomyces (plasmids or chromosomes). Two spontaneous mutant strains harboring TIRs of 480 and 850 kb were isolated. The TIR polymorphism seen is a result of the deletion of one chromosomal end and its replacement by 480 or 850 kb of sequence identical to the end of the undeleted chromosomal arm. Analysis of the wild-type sequences involved in these rearrangements revealed that a recombination event took place between the two copies of a duplicated DNA sequence. Each copy was mapped to one chromosomal arm, outside of the TIR, and encoded a putative alternative sigma factor. The two ORFs, designated hasR and hasL, were found to be 99% similar at the nucleotide level. The sequence of the chimeric regions generated by the recombination showed that the chromosomal structure of the mutant strains resulted from homologous recombination events between the two copies. We suggest that this mechanism of chromosomal arm replacement contributes to the rapid evolutionary diversification of the sequences of the TIR in Streptomyces.
栖土链霉菌DSM40697的染色体DNA是一个8兆碱基的线性分子，其末端为210千碱基的末端反向重复序列（TIRs）。在链霉菌的不同线性复制子（质粒或染色体）之间，TIRs的序列高度可变。分离出了两个带有480千碱基和850千碱基TIRs的自发突变菌株。观察到的TIR多态性是由于一条染色体末端缺失，并用与未缺失染色体臂末端相同的480或850千碱基序列进行替换的结果。对这些重排中涉及的野生型序列的分析表明，在一个重复DNA序列的两个拷贝之间发生了重组事件。每个拷贝都定位在一条染色体臂上，位于TIR之外，并编码一个假定的替代sigma因子。这两个开放阅读框，分别命名为hasR和hasL，在核苷酸水平上有99%的相似性。重组产生的嵌合区域的序列表明，突变菌株的染色体结构是由两个拷贝之间的同源重组事件导致的。我们认为，这种染色体臂替换机制有助于链霉菌中TIR序列的快速进化多样化。
