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. Author manuscript; available in PMC: 2012 Apr 4.
Published in final edited form as: Crit Rev Biochem Mol Biol. 2007 Sep-Oct;42(5):399–435. doi: 10.1080/10409230701648502

FIGURE 2.

FIGURE 2

The E. coli Lac assay for stress-induced mutagenesis. Above: schematic representation of the strain used in the Lac assay. This strain bears a ~200 kb F′ conjugative plasmid carrying the mutant lacIZ33 allele, a lacI-lacZ fusion gene with a +1 frameshift mutation. Below: generation-dependent mutation events occurring during growth of the culture prior to plating on lactose medium are detected as Lac+ colonies present on about day 2. During subsequent days of incubation, stress-induced Lac+ colonies accumulate, and include both Lac+ point mutants with a compensatory frameshift mutation or lac-amplified cells, with 20 to 50 copies of DNA spanning the lac gene, which provides sufficient beta-galactosidase activity for growth without a frameshift reversion mutation (Hastings et al., 2004).