title: DNA sequence context affects UV-induced mutagenesis in Escherichia coli.
abstract: We investigated the effect of altering the DNA sequence surrounding a mutable target site on the production of ultraviolet light (UV) induced mutations. Site-directed base substitutions were incorporated on both sides of a TAA sequence encoding a UAA nonsense defect in the tyrA14 allele of Escherichia coli. This allele is readily revertable by UV and a total of eight different base substitution mutations can be recovered. Five different strains harboring DNA sequences allowing the formation of 5'-TT, 5'-CT and 5'-TA* photoproducts were constructed and exposed to UV. DNA sequence analysis was used to determine the spectrum of the revertants that were recovered. The results showed that changes at the 3'-base of a TT site were predominantly T to C transitions and T to A transversions. However, unlike the TT site, a 5'-CT site produced a relatively high frequency of T to G transversions. In addition, T to A transversions that could not have been targeted by a cyclobutane-type or [6-4]-type pyrimidine dimer were produced; this result suggested that these mutations may be targeted by a TA* photoproduct. Also, a distinct strand bias was noted for two mechanistically identical base substitutions in a strain having a palindromic target sequence; this result may reflect an unequal damage distribution or processing of photoproducts as a consequence of asymmetric DNA replication. Finally, our results show that DNA sequences expected to allow the greatest density of UV-induced DNA damage produce the highest mutation frequencies. Overall, these findings provide new insights regarding the role of DNA photoproducts in UV mutagenesis.
authors.affiliation: Department of Microbiology, Southern Illinois University, Carbondale, IL 62901, United States.
authors.forename: Wai Bing
authors.initials: WB
authors.lastname: Mak
authors.forename: Douglas
authors.initials: D
authors.lastname: Fix
mesh_terms: D001483:Base Sequence / Q000528:radiation effects*; D004269:DNA, Bacterial / Q000528:radiation effects; D004926:Escherichia coli / Q000235:genetics* / Q000528:radiation effects*; D016296:Mutagenesis; D016297:Mutagenesis, Site-Directed; D014466:Ultraviolet Rays
chemical_list: D004269:DNA, Bacterial