Jackson E N, Yanofsky C
J Bacteriol. 1973 Oct;116(1):33-40. doi: 10.1128/jb.116.1.33-40.1973.
Mutants of Escherichia coli were selected in which a single mutational event had both relieved the polar effect of an early trpE mutation on trpB and simultaneously released the expression of trpB from tryptophan repression. The frequency at which these mutations appeared was roughly equal to the frequency of point mutations. In each of these mutants, the mutation increased the function of trpB and also increased the activity of some, but not all, of the other four tryptophan operon genes. Genetic analysis showed that the mutations were not located within the trp operon since in each case the parental trp operon could be recovered from the mutants. Each mutant was shown to carry a duplication of a trp operon segment translocated to a new position near the trp operon. Polarity is relieved since the trpB duplication-translocation is not in the same operon as the trpE polar mutation. The duplicated and translocated segments are fused to operons not regulated by tryptophan, so trpB function is no longer subject to tryptophan repression. The properties of the mutants indicate that the length of the duplicated segment and the position to which it is translocated differ in each of the seven mutants studied. The duplications are unstable, but the segregation pattern observed is not consistent with a single crossover model for segregation. That such duplication-translocation events generate a variety of new genetic arrangements at a frequency comparable with point mutations suggests they may play an important role in evolution.
挑选出了大肠杆菌的突变体，其中单个突变事件既能消除早期trpE突变对trpB的极性效应，又能同时使trpB的表达从色氨酸阻遏中释放出来。这些突变出现的频率大致与点突变的频率相等。在每一个这样的突变体中，该突变增强了trpB的功能，同时也增强了其他四个色氨酸操纵子基因中部分（而非全部）基因的活性。遗传分析表明，这些突变并不位于trp操纵子内，因为在每种情况下，亲代trp操纵子都可以从突变体中恢复。结果显示，每个突变体都携带了一个trp操纵子片段的重复，该片段易位到trp操纵子附近的一个新位置。由于trpB重复 - 易位与trpE极性突变不在同一个操纵子中，极性效应得以消除。重复并易位的片段与不受色氨酸调控的操纵子融合，因此trpB的功能不再受色氨酸阻遏。这些突变体的特性表明，在所研究的七个突变体中，每个突变体的重复片段长度及其易位的位置都有所不同。这些重复是不稳定的，但观察到的分离模式与用于分离的单交换模型不一致。这种重复 - 易位事件以与点突变相当的频率产生各种新的基因排列，这表明它们可能在进化中发挥重要作用。
