Chao L, Vargas C, Spear B B, Cox E C
Nature. 1983;303(5918):633-5. doi: 10.1038/303633a0.
Strains of the bacterium Escherichia coli harbouring genes that increase mutation rates are known to have an evolutionary advantage in chemostat competition over otherwise isogeneic strains with lower mutation rates. This advantage is frequency-dependent, the mutator strain being favoured only above a starting ratio of approximately 5 x 10(-5), and it results from the fact that the necessary beneficial mutations cannot be generated in a mutator population below a certain size. Here we consider the possibility that the mutagenic properties of transposable elements confer an advantage in the same manner as mutator genes. A previous report has shown that the transposon Tn5 increases the fitness of E. coli in chemostats, although the reason for this effect has not been established. Our results show that the transposon Tn10 also confers an advantage in chemostats. In addition, we find that (1) this advantage, like that associated with mutator genes, is frequency-dependent, (2) whenever the Tn10 strains win, a segment of Tn10, probably its IS10 sequences, has undergone transposition to a new site, (3) the new insertions converge into a site contained within a 3.2 kilobase (kb) PvuII fragment of the genome, and (4) no transpositions are detected when the Tn10 population loses. We conclude that Tn10 confers an advantage by increasing the mutation rate of the host bacterium.
已知携带可提高突变率基因的大肠杆菌菌株，在恒化器竞争中相对于其他具有较低突变率的同基因菌株具有进化优势。这种优势是频率依赖性的，突变菌株仅在起始比例约为5×10⁻⁵以上时才受到青睐，其原因是在低于一定规模的突变群体中无法产生必要的有益突变。在此，我们探讨转座元件的诱变特性是否以与突变基因相同的方式赋予优势。先前的一份报告显示，转座子Tn5可提高大肠杆菌在恒化器中的适应性，尽管尚未确定这种效应的原因。我们的结果表明，转座子Tn10在恒化器中也具有优势。此外，我们发现：（1）这种优势与突变基因相关的优势一样，是频率依赖性的；（2）每当Tn10菌株获胜时，Tn10的一段序列，可能是其IS10序列，会发生转座至新位点；（3）新的插入集中在基因组一个3.2千碱基（kb）PvuII片段内的一个位点；（4）当Tn10群体失败时未检测到转座现象。我们得出结论，Tn10通过提高宿主细菌的突变率赋予优势。
