Site and strand specificity of UVB mutagenesis in the SUP4-o gene of yeast.

Abstract

DNA sequencing was used to characterize 208 mutations induced in the SUP4-o tRNA gene of the yeast Saceharomyces cerevisiae by UVB (285-320 am) radiation. The results were compared to those for an analysis of 211 SUP4-o mutations induced by 254-nm UVC light. In each case, >90% of the mutations were single base-pair changes but GC -+ APT transitions predominated and accounted for more of the mutations induced by UVB than UVC. Double substitutions, single base-pair deletions, and more complex events were also recovered. However, UVB induced 3-fold more tandem substitutions than UVC and nontandem double events were detected only after irradiation with UVC. Virtually all induced substitutions occurred at sites where the pyrimidine of the base pair was part of a dipyrimidine sequence. Although the site specificities were consistent with roles for cyclobutane dimers and pyrimidinepyrimidone(6-4) lesions in mutation induction, preliminary photoreactivation data implicated cyclobutane dimers as the major form of premutational DNA damage for both agents.Intriguingly, there was a preference for both UVB-and UVCinduced mutations to occur at sites where the dipyrimidine was on the transcribed strand.Exposure to sunlight has been linked with the development of human skin cancer and experimental evidence indicates that among the wavelengths that penetrate the ozone layer, those responsible for carcinogenesis are within the UVB (280-320 nm) region ofthe solar spectrum (1-3). Although the precise mechanism is unknown, DNA damage, as expressed by genetic mutation, is very likely to be involved in the induction of cancer by sunlight. Indeed, the human disorder xeroderma pigmentosum involves defective repair of UVinduced DNA lesions and sensitizes affected individuals to solar photocarcinogenesis (4). Furthermore, sunlight has been shown to mutagenize Chinese hamster cells (5) and UVB wavelengths are mutagenic to bacterial, yeast, and mammalian cells (6-8).It is generally considered that cyclobutane pyrimidine dimers and pyrimidine-pyrimidone(6-4) photoproducts [(6-4) photoproducts] are the most important premutational DNA lesions induced by UVC (200-280 nm) radiation (9, 10). Although UVB wavelengths also produce these two types of photoproduct (6,(10)(11)(12), other lesions such as DNA strand breaks and thymine glycols are induced by UVB treatment (13,14). The precise roles of these various lesions in UVB mutagenesis, and thus the mechanism(s) involved, have yet to be established. There is evidence implicating cyclobutane dimers and (6-4) photoproducts in the mutagenicity of UVB radiation (12, 15), but there also are indications that the nondimer lesions may contribute significantly (6, 15). To add to the complexity of this situation, other factors may influence the production of UVB-induced mutations. For exampie, UVC-induced cyclobutane dimers are selectively repaired in the transcribed strand of transcriptionally active dhfr, lacI, or URA3 genes in human and rodent, bacterial, or yeast cells, respectively (16)(17)(1...

Near-Ultraviolet Mutagenesis in Superoxide Dismutase-deficient Strains of Escherichia coli.

“…Furthermore, the A:T to T:A transversion was weakly stimulated along with all of the frameshifts analyzed. The finding that G:C to A:T transitions predominate near-UV-induced base substitutions correlates with other studies of ultraviolet radiation mutagenesis (29,30). Armstrong ed site and strand specificity in the SUP4-o gene in S. cerevisiae after UV-B (285-320 nm) mutagenesis.…”

“…This has been interpreted to indicate that cyclobutane pyrimidine diners and pyrimidinepyrimidone (6-4) photoproducts are the most important premutational lesions induced by far-UV radiation. However, cyclobutane dimers have recently been implicated as the major form of premutational DNA damage for both far-UV and mid-UV radiations (30). It should be noted that there is some region of overlap in the mid-UV (285-320 nm) radiation studied by Armstrong and Kunz (30) and the near-UV (300-420 nm) radiation used in these experiments.…”

Highly error-free role of DNA polymerase η in the replicative bypass of UV-induced pyrimidine dimers in mouse and human cells

“…In striking contrast, a remarkably distinct pattern of UV-induced mutations is observed upon Pol vs. Rev3 knockdown, given that a very different set of hotspots remain after the knockdown of Pol (nos. 4,5,7,8) vs. the knockdown of Rev3 (nos. 1, 2, 3, 6); whereas the incidence of mutations at hotspot positions 9-11 is greatly diminished upon the knockdown of either Pol or Rev3 (Fig.…”

“…In addition to the formation of cyclobutane dimers at TT sites, UV induces the formation of dimers at 5Ј-TC-3Ј and 5Ј-CC-3Ј dipyrimidine sites, and both in yeast and humans, UV-induced mutations occur predominantly by a C-to-T transition at the 3Ј base (5)(6)(7)(8). To obtain a more comprehensive understanding of the error-free vs. mutagenic roles of TLS Pols in promoting replication through CPDs formed at various dipyrimidine sequences, we have analyzed UV mutagenesis in the cII transgene carried in a mouse cell line that additionally harbors a (6-4) PP photolyase gene so that (6-4) photoproducts can be selectively removed.…”

Requirement of DNA Polymerase η for Error-Free Bypass of UV-Induced CC and TC Photoproducts

“…In addition to cyclobutane dimers at two adjacent thymines, UV also induces the formation of lesions at dipyrimidine sites that involve a cytosine, most commonly at 5Ј-TC-3Ј and 5Ј-CC-3Ј sequences. In fact, the 3Ј cytosine in both sequence contexts is highly mutagenic, and in both yeast and humans, UV-induced mutations occur primarily by a C3T transition that would result from the insertion of an A opposite the 3Ј damaged C residue during DNA replication (1,3,6). If Pol were an A rule polymerase which inserts an A residue by default opposite the various lesions, then the bypass of a CC or a TC cyclobutane dimer by Pol would be mutagenic, not error free as for the TT dimer.…”