Mutational Specificity of UV Light in E. Coli: Influence of Excision Repair and the Mutator Plasmid PKM101

Abstract

The lacI system of E. coli provides a method for determining UV-induced mutational specificity at a large number of sites (1,2,3). In contrast, earlier studies in other systems have generally relied upon the analysis of reversion at a rather limited number of sites (4,5,6). Often, the mutants analyzed in reversion studies were originally induced by the mutagenic treatment (7) and the possibility therefore exists that preferentially mutable sites or hotspots were selected and that these may have behaved atypically. Alternatively, the original mutation may have removed a DNA sequence target and these sites may be in fact partially immutable! Moreover, in studies of the reversion of nonsense mutations, the majority of “revertants” actually occur not in the structural gene but at suppressor loci which behave unusually in their response to UV light (4,8,9). The lacI system allows the examination of forward mutagenesis at 65^† individual sites where nonsense mutations can arise by a single base substitution. Since both the DNA sequence and the location of the nonsense mutations have been established (10), each mutation can be attributed to a specific transition or transversion event.