Abdulkarim F, Hughes D
Department of Molecular Biology, Uppsala University, Sweden.
J Mol Biol. 1996 Jul 26;260(4):506-22. doi: 10.1006/jmbi.1996.0418.
The genes coding for the translation factor EF-Tu, tufA and tufB are separated by over 700 kb on the circular chromosome of Salmonella typhimurium. The coding regions of these genes have 99% identity at the nucleotide level in spite of the presumed ancient origin of the gene duplication. Sequence comparisons between S. typhimurium and Escherichia coli suggest that within each species the two tuf genes are evolving in concert. Here we show that each of the S. typhimurium tuf genes can transfer genetic information to the other. In our genetic system the transfers are seen as non-reciprocal, i.e. as gene conversion events. However, the mechanism of recombination could be reciprocal, with sister chromosome segregation and selection leading to the isolation of a particular class of recombinant. The amount of sequence information transferred in individual recombination events varies, but can be close to the entire length of the gene. The recombination is RecABCD-dependent, and is opposed by MutSHLU mismatch repair. In the wild-type, this type of recombination occurs at a rate that is two or three orders of magnitude greater than the nucleotide substitution rate. The rate of recombination differs by six orders of magnitude between a recA and a mutS strain. Mismatch repair reduces the rate of this recombination 1000-fold. The rate of recombination also differs by one order of magnitude depending on which tuf gene is donating the sequence selected for. We discuss three classes of model that could, in principle, account for the sequence transfers: (1) tuf mRNA mediated recombination; (2) non-allelic reciprocal recombination involving sister chromosomes; (3) non-allelic gene conversion involving sister chromosomes, initiated by a double-strand break close to one tuf gene. Although the mechanism remains to be determined, the effect on the bacterial cells is tuf gene sequence homogenisation. This recombination phenomenon can account for the concerted evolution of the tuf genes.
编码翻译因子EF-Tu的基因tufA和tufB在鼠伤寒沙门氏菌的环状染色体上相隔超过700 kb。尽管推测这种基因重复起源古老，但这些基因的编码区在核苷酸水平上有99%的同一性。鼠伤寒沙门氏菌和大肠杆菌之间的序列比较表明，在每个物种中，两个tuf基因是协同进化的。在这里我们表明，鼠伤寒沙门氏菌的每个tuf基因都可以将遗传信息传递给另一个基因。在我们的遗传系统中，这种传递被视为非相互的，即作为基因转换事件。然而，重组机制可能是相互的，通过姐妹染色单体分离和选择导致特定一类重组体的分离。在单个重组事件中传递的序列信息量各不相同，但可能接近基因的全长。这种重组是RecABCD依赖性的，并受到MutSHLU错配修复的抑制。在野生型中，这种类型的重组发生的速率比核苷酸替换速率高两到三个数量级。recA和mutS菌株之间的重组速率相差六个数量级。错配修复将这种重组速率降低了1000倍。根据哪个tuf基因提供被选择的序列，重组速率也相差一个数量级。我们讨论了原则上可以解释序列转移的三类模型：(1) tuf mRNA介导的重组；(2) 涉及姐妹染色单体的非等位基因相互重组；(3) 涉及姐妹染色单体的非等位基因转换，由靠近一个tuf基因的双链断裂引发。尽管机制仍有待确定，但对细菌细胞的影响是tuf基因序列的同质化。这种重组现象可以解释tuf基因的协同进化。
