Evidence that gene amplification underlies adaptive mutability of the bacterial lac operon.

Adaptive mutability is the apparent alteration in specificity or rate of mutability seen in bacteria during stress. A model is proposed by which gene amplification during selective growth can give the appearance of adaptive mutability without requiring any change in mutability. The model is based on two assumptions, that a mutant lac locus with residual function allows growth if its copy number is increased, and that true reversion events are made more likely by replication of chromosomes with many copies of the locus. Apparent directed mutability, its recombination requirement, and its apparent independence of cell growth are all accounted for by the model. Evidence is provided for the required residual function and gene amplification.