Adaptive evolution that requires multiple spontaneous mutations. I. Mutations involving an insertion sequence.
Adaptive evolution that requires multiple spontaneous mutations. I. Mutations involving an insertion sequence.
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作者：
B. Hall
大肠杆菌K12菌株chi 342 LD需要bgl（β-葡糖苷酶）操纵子中的两个突变bglR 0-bglR+和从bglF内切除IS 103，以便利用水杨苷。在生长的细胞中，这两种突变分别以每次细胞分裂4 × 10（-8）和每次细胞分裂小于2 × 10（-12）的速率发生。在MacConkey水杨苷平板上2-3周龄的菌落中，双突变体以每个细胞10（-8）的频率发生，但对缬氨酸的抗性突变率不受影响。这两个突变是顺序发生的。8-12天龄的菌落含有1%至约10%的IS 103切除突变体，由此产生Sal+次级bglR 0-bglR+突变体。它表明，切除突变体是不有利的殖民地内，因此，它们必须导致从一个独立的切除后，在生活中的殖民地的爆发。IS 103的切除仅发生在含有水杨苷的培养基上，尽管事实上切除本身并不赋予可检测的选择性优势，并且仅用于产生二次选择性有利突变的可能性。
Escherichia coli K12 strain chi 342LD requires two mutations in the bgl (beta-glucosidase) operon, bglR0----bglR+ and excision of IS103 from within bglF, in order to utilize salicin. In growing cells the two mutations occur at rates of 4 x 10(-8) per cell division and less than 2 x 10(-12) per cell division, respectively. In 2-3-week-old colonies on MacConkey salicin plates the double mutants occur at frequencies of 10(-8) per cell, yet the rate of an unselected mutation, resistance to valine, is unaffected. The two mutations occur sequentially. Colonies that are 8-12 days old contain from 1% to about 10% IS103 excision mutants, from which the Sal+ secondary bglR0----bglR+ mutants arise. It is shown that the excision mutants are not advantageous within colonies; thus, they must result from a burst of independent excisions late in the life of the colony. Excision of IS103 occurs only on medium containing salicin, despite the fact that the excision itself confers no detectable selective advantage and serves only to create the potential for a secondary selectively advantageous mutation.