Chapter 4 – SOLAR UV IRRADIATION AND THE GROWTH AND DEVELOPMENT OF HIGHER PLANTS

The (6-4) photoproduct is an important determinant of the lethal and mutagenic effects of UV irradiation of biological systems. The removal of this lesion appears to correlate closely with the early DNA repair responses of mammalian cells, including DNA incision events, repair synthesis and removal of replication blocks. The processing of (6-4) photoproducts and cyclobutane dimers appears to be enzymatically coupled in bacteria and most mammalian cell lines examined (i.e. a mutation affecting the repair of one lesion also often affects the other), although exceptions exist in which repair capacity may be evident for one photoproduct and not the other (e.g. UV61 and the XP revertant cell line). These differences in the processing of the two photoproducts in some cell lines of human and rodent origin suggest that in mammalian cells, different pathways for the repair of (6-4) photoproducts and cyclobutane dimers may be used. This observation is further supported by pleiotropic repair phenotypes such as those observed in CHO complementation class 2 mutants (e.g., UV5, UVL-1, UVL-13, and V-H1). Indirect data, from HCR of UV irradiated reported genes and the cytotoxic responses of UV61, suggest that the (6-4) photoproduct is cytotoxic in mammalian cells and may account for 20 to 30% of the cell killing after UV irradiation of rodent cells. Cytotoxicity of the (6-4) photoproduct may be important in the etiology of sunlight-induced carcinogenesis, affecting mutagenesis as well as tumorigenesis. The intricate photochemistry of the (6-4) photoproduct, its formation and photoisomerization, is in itself extremely interesting and may also be relevant to sunlight carcinogenesis. The data reviewed in this article support the notion that the (6-4) photoproduct and its Dewar photoisomer are important cytotoxic determinants of UV light. The idea that the (6-4) photoproduct is an important component in the spectrum of UV-induced cytotoxic damage may help clarify our understanding of why rodent cells survive the effects of UV irradiation as well as human cells, without apparent cyclobutane dimer repair in the bulk of their DNA. The preferential repair of cyclobutane dimers in essential genes has been proposed to account for this observation (Bohr et al., 1985, 1986; Mellon et al., 1986). The data reviewed here suggest that understanding the repair of a prominent type of noncyclobutane dimer damage, the (6-4) photoproduct, may also be important in resolving this paradox.

The biology of the (6-4) photoproduct.

Thirty-one metal salts have been tested for their ability to affect the accuracy of DNA synthesis in vitro. All ten salts of metal carcinogens decreased the fidelity of DNA synthesis. Of the three metals which beforehand were considered to be possible mutagens or carcinogens, only one decreased fidelity. In contrast, 17 noncarcinogenic metal salts did not affect fidelity even when present at concentrations that were clearly inhibitory.

https://science.sciencemag.org/content/sci/194/4272/1434.full.pdf

Infidelity of DNA synthesis in vitro: screening for potential metal mutagens or carcinogens.

The frequency of UV-induced mutations at specific nucleotides along the lacI gene of Escherichia coli varies by as much as 80-fold1. The spectrum of mutations to amber, ochre, and UGA chain-terminating codons includes five hotspots accounting for over 30% of these UV-induced nonsense mutations1. The simplest explanation for the hotspot phenomenon is that the mutation frequency reflects the frequency of UV-induced DNA damage at the mutation site. To test this possibility, we measured the distribution, in a 380-base pair (bp) region of the lacI gene containing the nonsense mutation hotspots, of UV-induced cyclobutane pyrimidine dimers and pyrimidine–pyrimidine (6-4) photoproducts. The latter (also called the PydC lesion) is a UV-induced DNA lesion which occurs predominantly at TC and CC sequences2. The results reported here reveal the presence of UV-induced base damage hotspots which include the nonsense mutation hotspots. There is a linear relationship between base damage incidence and mutation incidence. The results suggest that the UV-induced pyrimidine–pyrimidine (6-4) lesion may be mutagenic.

UV-induced mutation hotspots occur at DNA damage hotspots

Publisher Summary This chapter is concerned with the structures and interactions of purines and pyrimidines in the crystalline state. However, because of their close similarity to the pyrimidines, it also discusses the crystal structure of the barbiturates. The chapter reviews the structure arid conformation of the sugar and the phosphate components of nucleosides, nucleotides, and nucleic acids, and their steric relationship to the purines and pyrimidines. It discusses relevance of single-crystal analysis of purine–pyrimidine interactions to the fiber analysis of polynucleotides. The chapter provides an overview on X-ray diffraction method. It presents a complete tabulation of the molecular geometry of the reliable structures for which there is three-dimensional diffraction information. It draws the conclusion that the structures of the individual purine and pyrimidine bases retain a fair amount of constancy in a variety of different crystal lattices. The chapter also focuses on the H-bonds that are formed between the purines and pyrimidines. In contrast to the narrow range of covalent bond lengths that are found within the molecules, there is considerable variation in both the length of H-bonds and their angles, a point that is emphasized in this presentation. As there is an appreciable range in the geometry of H-bonded interactions, it is likely that a greater variety of stable structures can be formed. An important variable in this connection is the variation in H-bond lengths, as the length of this bond characterizes the energy of H-bonding interactions in part. It reviews that purine and pyrimidine crystalIography underlines the diversity of the H-bonding potentiality of the nucleic acid bases; it is likely that this diversity has been utilized fully by nature in the evolution of the information-transferring system.

The crystal structures of purines, pyrimidines and their intermolecular complexes.

A product isolated from thymine irradiated with ultraviolet light in frozen aqueous solution undergoes dehydration on heating with acids. As judged by elemental analysis, mass, ultraviolet, infrared, and nuclear magnetic resonance spectra, the most probable structures for this compound and its dehydration product, respectively, are 5-hydroxy-6-4'-[5''-methylpyrimidin-2'-one]-dihydrothymine and 6-4'-[5'-methylpyrimidin-2'-one]-thymine. Apparently, this compound is a thymine-thymine adduct and presumably is formed through the rearrangement of an initial photoproduct. Both compounds are closely related to 6-4'-[pyrimidin-2'-one]-thymine which has been isolated from acid hydrolyzates of ultraviolet-irradiated DNA and supposedly is derived from cytosine-thymine adduct. Formation of such adducts between pyrimidine bases is apparently a common photoreaction and may be important to the study of the photochemistry and photobiology of nucleic acids.

https://science.sciencemag.org/content/sci/160/3824/186.full.pdf

Thymine-Thymine Adduct as a Photoproduct of Thymine

A new thymine-derived product was separated from DNA irradiated with utlraviolet light in vitro and in vivo. This compound was mistaken to be thymine homodiner (T=T) by other workers because it is chromatographically indistinguishable from T=T in most eluents. It has absorbancy maximums at 312, 312, and 300 millimicrons in neutral, pH 2, and pH 11 aqueous solutions, respectively. When it is irradiated in aqueous solution with wavelengths of 360 and 313 millimicrons its spectrum reverts to one similar to that of thymine. Therefore, at least three thymine-derived products can be detected in ultraviolet irradiated DNA, namely the homodimer, a material with absorbancy maximum at 312 millimicrons, and a "minor" product suggested by others to be a dimer of cytosine and thymine. In cells, the latter two are formed in aboult equal amounts. While these three products were shown to exist in the acid hydrolyzates of ultraviolet irradiated DNA, a material with absorbancy maximum at about 310 millimicrons was demonstrated to form in ultraviolet irradiated DNA without further treatment. The magnitude of this spectral increase varied directly with the incrcase in the adenine-thymine contents in the DNA as shlown by differential transmittance spectra of the irradiated Micrococcus lysodeikticus, calf thymus, Bacillus cereus, and Hemophilus influenzae DNA.

https://science.sciencemag.org/content/sci/156/3777/955.full.pdf

Ultraviolet irradiation of dna in vitro and in vivo produces a third thymine-derived product.

Thymine-thymine adduct is a product isolated from thymine irradiated with ultraviolet light in frozen aqueous solution. This compound is presumably formed through the rearrangement of an initial photoproduct. Single crystal x-ray diffraction analysis has confirmed the molecular formula of the adduct, 5-hydroxy-6-49-(59-methylpyrimid-29-one)-dihydrothymine, except for the possibility of a hydrogen atom on the 39 nitrogren rather than the 19 nitrogen, and has established the stereoconfiguration of the molecule.

https://science.sciencemag.org/content/sci/164/3876/183.full.pdf

Crystal and Molecular Structure of a Thymine-Thymine Adduct

Two new products were isolated from uracil irradiated with ultraviolet light in frozen aqueous solution. As judged by mass, nuclear magnetic resonance, and ultraviolet and infrared spectra, one is a photopolymer, U 3 and the structure of the other is probably 6-49-[ pyrimidin-29-one]-uracil. Formations of these products between pyrimidine bases are apparently common photoreactions, and may be important to the study of the photochemistry and photobiology of nucleic acids.

http://science.sciencemag.org/content/sci/163/3873/1341.full.pdf

Uracil photoproducts from uracil irradiated in ice.

WE describe the mutagenic action of chemically synthesised cis-5,6-dihydro-6-hydroperoxy-5-hydroxythymine (6-TOOH)1 on transforming DNA of Haemophilus influenzae2,3, and suggest a mechanism by which ionising radiation induces mutations in vivo.

S. Y. Wang

Papers

Thymine-Thymine Adduct as a Photoproduct of Thymine

Ultraviolet irradiation of dna in vitro and in vivo produces a third thymine-derived product.

Crystal and Molecular Structure of a Thymine-Thymine Adduct

Uracil photoproducts from uracil irradiated in ice.

Thymine hydroperoxide as a mediator in ionising radiation mutagenesis