FIG. 1.

Multiple events precede reversion. A preexisting cell with a lac duplication initiates a clone on selective medium. Growth is slow but improves as selection favors cells with more lac copies. Reversion does not require mutagenesis. Selection then favors lac⁺ cells that lose mutant copies. Ultimately a lac⁺ haploid segregant arises and overgrows the clone. During growth, single-stranded DNA, produced from the plasmid transfer (tra) origin, induces SOS. General mutagenesis occurs in those clones whose lac amplification includes the dinB⁺ gene.