Figure 5.

How lac amplification enhances revertant yield without mutagenesis. Each act of lac replication provides an opportunity for a reversion event (frameshift). A cell with only one lac copy has one opportunity to revert for each cell division or plasmid replication (see left column). As the lac allele amplifies, each cell gains additional chances for a reversion event with no increase in mutation rate (right column). If the amplified array improves growth on lactose, then the lineage on the right expands faster than that on the left (without an amplification) and this growth also adds to the likelihood of a reversion event. In effect, amplification directs mutations to the exact base pairs that limit growth, because once a revertant allele forms, selection holds only that allele, whereas the nonrevertants alleles are no longer selectively retained. The likelihood of an unselected mutation near lac is the same with and without selection, because only the revertant allele is kept in the genome.