DNA sequence context affects UV-induced mutagenesis in Escherichia coli.
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.
Mak
Wai Bing
WB
Department of Microbiology, Southern Illinois University, Carbondale, IL 62901, United States.
Fix
Douglas
D
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
D004269:DNA, Bacterial