Figure 1
Expansion of a chromosomal duplication. A. Chromosomal (lacI33-lacZ)-lacY [38] was duplicated by phage λ Red-mediated recombination with a linear DNA bearing homology-flanked antibiotic resistance marker Ab. A hypothetical mechanism by which the duplication could be generated, involving crossovers between the linear DNA and both copies of the replicating chromosomal target, is diagrammed [9]. The duplication was constructed with a tetracycline resistance element, which was later replaced with cat. Under selection for Lac function, the (lacIZ33Y)2-cat duplication expands into multiple copies. L and R – chromosomal sequences flanking lac. E – EcoR1 restriction sites. B. Multiple copies of the repeated sequence are seen as bands produced by EcoR1 digestion of cellular DNA. Tests of two Lac+ revertants, one without (-), and one with (+) an expanded lac array, are shown as examples.