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PubMed1993

Effects of the K-shell X-ray absorption of phosphorus on the scission of the pentadeoxythymidylic acid.

The scission of pentadeoxythymidylic acid, d(pT)5, by monochromatic soft X-rays on (2153 eV) and below (2147 eV) the K-shell absorption peak of phosphorus was studied as a model of strand breakage in DNA. Samples dried on glass plates were irradiated by monochromatic soft X-rays in vacuo, and the products were analysed by HPLC. The main products, in ascending order of the retention time, were thymine, 5'-dTMP, d(pT)2, d(pT)3, d(pT)4, d(pTp) and three unknown products (UK 1, 2 and 3), which were presumed to be d(pTpTp), d(pTpTpTp) and d(pTpTpTpTp), respectively. No difference between 2147 and 2153 eV irradiation in the nature of the induced products was detected, indicating that the K-shell absorption of phosphorus and the following Auger process produced the same types of products as those produced by photoabsorption at the other shells of phosphorus and at other atoms. The cross-sections for the induction of products at 2153 eV were 3.3-4.0 times larger than those at 2147 eV, the ratios of these values being scattered around the ratio (3.65) of absorbance of the sample between 2153 and 2147 eV. The dependence on the X-ray energy, however, almost disappeared after conversion from exposure to absorbed dose; the ratios of the G-values (number of products per 100 eV) of the products were 0.92-1.11. Photoabsorption at the K-shell of phosphorus induced products comparable or slightly less effectively than photoabsorption at the others. These results indicate that the K-shell absorption of phosphorus and the following Auger process do not have any characteristic effect on strand breakage in dry DNA, either qualitatively or quantitatively.

Absorption↗
PubMed1995

Multiple pathways for homologous recombination in Saccharomyces cerevisiae.

The genes in the RAD52 epistasis group of Saccharomyces cerevisiae are necessary for most mitotic and meiotic recombination events. Using an intrachromosomal inverted-repeat assay, we previously demonstrated that mitotic recombination of this substrate is dependent upon the RAD52 gene. In the present study the requirement for other genes in this epistasis group for recombination of inverted repeats has been analyzed, and double and triple mutant strains were examined for their epistatic relationships. The majority of recombination events are mediated by a RAD51-dependent pathway, where the RAD54, RAD55 and RAD57 genes function downstream of RAD51. Cells mutated in RAD55 or RAD57 as well as double mutants are cold-sensitive for inverted-repeat recombination, whereas a rad51 rad55 rad57 triple mutant is not. The RAD1 gene is not required for inverted-repeat recombination but is able to process spontaneous DNA lesions to produce recombinant products in the absence of RAD51. Furthermore, there is still considerably more recombination in rad1 rad51 mutants than in rad52 mutants, indicating the presence of another, as yet unidentified, recombination pathway.

Chromosome Inversion↗