Thymine Photoproducts but not Thymine Dimers Found in Ultraviolet-Irradiated Bacterial Spores

Bacillus megateriunm spores labeled with tritiated thymidine were irradiated with monochromatic ultraviolet light, and the DNA of the spores was analvzed for thymine-containing, products. No thymine dimers were observed, but three other thymine photoproducts were found. The unknown products of radiation were produced in vitro by irradiation of DNA that had been dried in the, presence of various salts.

Cysteamine protection of SCEs induced by UV and fluorescent light

CHO cells were grown for 36 h in the presence of BUDR. 12 h before harvesting, the cells were irradiated with UV or with fluorescent (FL) light. Part of the cultures were treated with cysteamine (Cys) during irradiation. Metaphase spreads were stained by the Hoechst-plus-Giemsa method, and the frequency of SCEs per chromosome was determined in second-mitosis cells. The basal frequency of SCEs (1.07 +/- 0.073) was not increased by Cys treatment (1.15 +/- 0.048). On the other hand, UV and FL induced a 3-4-fold increase in the frequency of exchanges (4.18 +/- 0.2 and 3.32 +/- 0.12 resp.). Cys, when present during irradiation, markedly reduced the frequency of UV- and FL-induced SCEs (2.7 +/- 0.14 and 2.29 +/- 0.11 resp.). It is assumed that Cys prevents the formation of radiation-induced breakages in the BUDR-substituted DNA. The residual increase over the basal levels of SCEs which remains after the treatment with Cys may be due to the persistence of some open breaks, to the presence of protein-DNA crosslinking, or to the combined action of both types of lesion.

The lesions produced by ultraviolet light in DNA containing 5-bromouracil

The compound 5-bromouracil (BrU) may be incorporated into DNA in place of its analog thymine. This review is concerned with the photochemical lesions produced by the action of ultraviolet light on such BrU-DNA, and consequent biological effects of such lesions.

Photoinduced DNA-protein cross-links and bacterial killing: a correlation at low temperatures

The increased sensitivity of Escherichia coli to killing by ultraviolet irradiation when frozen and the variation in this sensitivity as a function of the temperature during irradiation have been correlated with changes in the amount of DNA that was cross-linked to protein by ultraviolet light. These variations in sensitivity to killing do not correlate with the production of thymine dimers.

Cyclobutane-type pyrimidine dimers in polynucleotides

The formation of cyclobutane-type dimers between adjacent pyrimidine residues in model polynucleotides or DNA may be represented by the general scheme See pdf 379.pdf Whereas the formation of all other known photoproducts follows the irreversible path See pdf 379.pdf Thus dimers are distinguished from other photoproducts by the fact that they can be monomerized, as well as formed, by ultraviolet irradiation. At large incident fluxes of photons the steady-state value of dimers depends on wavelength and pH, as well as on other characteristics of the surrounding medium. The number of dimers in an irradiated polynucleotide may be decreased by purely photochemical means, whereas this is not true for most other photoproducts, for which continued irradiation, irrespective of wavelength, always results in the formation of more photoproduct (37). The wavelength dependence of the steady-state for dimers is also reflected in the biological activity of irradiated transforming DNA. This experiment and the fact that photoreactivating enzyme plus visible light monomerizes dimers (and has not been demonstrated to have any effect on other photoproducts) are the strongest lines of experimental evidence that pyrimidine dimers of the cyclobutane type are biologically important lesions and can account for a large fraction of the effects of ultraviolet light on DNA in solution. Insofar as DNA is one of the more important biological structures, such dimers, when formed, account for a large part of the effects of ultraviolet radiation on biological systems.