title: CHO mutant UV61 removes (6-4) photoproducts but not cyclobutane dimers.
abstract: The CHO mutant UV61 was previously assigned to complementation group 6 of UV-sensitive rodent cell mutants. UV61 is less sensitive to killing by UV radiation than mutants such as UV5, which is highly defective in the incision process that acts on UV-induced lesions. The D37 for cell survival is approximately 4 J/m2 for UV61, compared with 10 J/m2 for the parental AA8 line and approximately 2 J/m2 for UV5. Similarly, mutation induction at the hprt and aprt loci shows an intermediate response to UV61. In a post-replication recovery assay, the kinetics of maturation of pulse-labelled nascent DNA were normal after UV irradiation in UV61. Data from alkaline elution and alkaline unwinding assays showed that the rates of break accumulation and resealing, measured 0-120 min after irradiation, were also normal in the mutant. This repair incision correlated with the rapid, normal removal of pyrimidine(6-4)pyrimidone photoproducts in UV61 measured using a radioimmunoassay that is specific for this class of damage. In contrast, after exposure to 10 or 15 J/m2, no detectable removal of cyclobutane dimers from DNA was found in UV61 while AA8 cells removed 32% by 24 h. We suggest that the mutation in UV61 specifically lowers the affinity of a repair protein for cyclobutane dimers, which are also inefficiently removed from the bulk DNA of normal CHO cells. The resistance of UV61 to killing by the direct acting chemical 7-bromomethylbenz[a]anthracene was only slightly greater than that of UV5, indicating defective repair of bulky chemical adducts in addition to cyclobutane dimers.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Biomedical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550.
authors.forename: L H
authors.initials: LH
authors.lastname: Thompson
authors.forename: D L
authors.initials: DL
authors.lastname: Mitchell
authors.forename: J D
authors.initials: JD
authors.lastname: Regan
authors.forename: S D
authors.initials: SD
authors.lastname: Bouffler
authors.forename: S A
authors.initials: SA
authors.lastname: Stewart
authors.forename: W L
authors.initials: WL
authors.lastname: Carrier
authors.forename: R S
authors.initials: RS
authors.lastname: Nairn
authors.forename: R T
authors.initials: RT
authors.lastname: Johnson
mesh_terms: D000818:Animals; D002470:Cell Survival / Q000528:radiation effects; D002478:Cells, Cultured; D002869:Chromosome Aberrations; D006224:Cricetinae; D003503:Cyclobutanes / Q000378:metabolism; D004260:DNA Repair / Q000528:radiation effects; D009154:Mutation; D014466:Ultraviolet Rays
chemical_list: D003503:Cyclobutanes