Specificity of mutation by UV light and delayed photoreversal in umuC-defective Escherichia coli K-12: a targeting intermediate at pyrimidine dimers

Abstract

Prototrophic mutants produced by UV light in Escherichia coli K-12 strains with argE3(0c) and hisG4(0c) defects are distinguished as backmutations and specffic nonsense suppressor mutations. In strains carrying a umuC defect, mutants are not produced unless irradiated cells are incubated and then exposed to photoreversing light (delayed photoreversal mutagenesis). The mutants thus produced are found to be specifically suppressor mutations and not backmutations. The suppressor mutations are primarily glutamine tRNA ochre suppressor mutations, which have been attributed previously to mutation targeted at T = C pyrimidine dimers.In a lexASi recA441 strain, where the SOS mutagenesis functions are constitutive, targeting at dimers is confirmed by demonstrating that the induction of glutamine tRNA suppressor mutations is susceptible to photoreversal. In the same strain induction of backmutations is not susceptible to photoreversal. Thus delayed photoreversal mutagenesis produces suppressor mutations that can be targeted at pyrimidine dimers and does not produce backmutations that are not targeted at pyrimidine dimers. This correlation supports the idea that delayed photoreversal mutagenesis in umuC defective cells reflects a mutation process arrested at a targeting pyrimidine dimer photoproduct, which is the immediate cause of both the alteration in DNA sequence and the obstruction (unless repaired) to mutation fixation and ultimate expression.

An in vivo complex with DNA photolyase blocks UV mutagenesis targeted at a thymine-cytosine dimer in Escherichia coli

“…These suppressor mutations have been shown to arise from C-to-T transitions at DNA sites affecting the anticodon, i.e., -ATCAAAA-to -ATTAAAA-(the transcribed strand encoding the anticodon loop) (5). Either T=C or T(6-4)C photoproducts could form at this mutation site, and targeting by a variable proportion of T=C and T(6-4)C has been proposed (1,2). Back mutations at the ochre defect (-TTA-in the transcribed strand) apparently are not targeted by photoreversible dimers (1,14).…”

“…Previously the specific recognition of photolyase for dimers was used to associate particular mutations and dimers by PR that removed the photoproduct and the related mutation (2,14). Now it seems possible to use an amplified level of photolyase (in the dark) and the same specific recognition to actively interfere with the mutagenesis process at a given damage site.…”

“…Only the response associated with a mutation targeted by a thymine-cytosine pyrimidine dimer was reduced in the dark in cells with amplified DNA photolyase. This specific reduction is attributed to the interruption of mutational DNA synthesis by a photolyase complex at the targeting dimer.The thymine-cytosine cyclobutane dimer (T=C) has been implicated as the UV photoproduct at the site of mutagenesis causing (targeting) C-to-T transitions in certain DNA sequences in Escherichia coli (2, lOa,12). DNA photolyase is a cellular protein that specifically binds to cyclobutanepyrimidine dimers (17), the major photoproduct produced in DNA after exposure to germicidal UV light (13).…”

“…The thymine-cytosine cyclobutane dimer (T=C) has been implicated as the UV photoproduct at the site of mutagenesis causing (targeting) C-to-T transitions in certain DNA sequences in Escherichia coli (2,lOa,12). DNA photolyase is a cellular protein that specifically binds to cyclobutanepyrimidine dimers (17), the major photoproduct produced in DNA after exposure to germicidal UV light (13).…”

Inactivation of E. Coli cell viability and DNA Photo‐breakage by Pulsed Nitrogen Laser Radiation

“…(3) At wave length above 320 nm the radiation effect on DNA has a mediated most probably, a photodyna-mical character. (4) The present investigation were carried out with the aim to study the effect of pulsed Nitrogen laser radiation on the viability of E. Coli cells and on its genomic content.…”