[Effect of nitrogen mustard on the DNA template activity in RNA synthesis]

Dissociation of malondialdehyde mutagenicity in Salmonella typhimurium from its ability to induce interstrand DNA cross-links

Malondialdehyde (MDA), an in vivo metabolite of lipid peroxidation and prostaglandin biosynthesis, is mutagenic in Salmonella typhimurium. It is a reactive electrophile that can form interstrand cross-links in DNA. To explore the possibility that MDA-induced interstrand cross-links are the pre-mutagenic lesion, we have quantitated the ability of highly purified preparations of MDA to form interstrand cross-links when reacted with linear plasmid DNA. At physiological temperature and pH, MDA did not form DNA cross-links as determined by DNA denaturation followed by agarose gel electrophoresis. DNA cross-links were formed, however, when incubations with MDA were carried out at either pH 4.2 or temperatures exceeding 60 degrees. alpha-Methylmalondialdehyde (CH3MDA) was found to cross-link DNA more efficiently than MDA, but was not mutagenic in any tester strain of Salmonella. MDA polymers, formed by acid incubation of MDA, also were capable of inducing cross-links. However, an inverse relationship was observed between mutagenicity and extent of polymerization. The pattern of mutagenic response for MDA in different strains of Salmonella was compared with mitomycin C, an established mutagenic cross-linking agent. Error-prone repair and a UvrB+ phenotype, which are needed for the induction of mutations by mitomycin C, were not required for MDA mutagenesis. These findings, taken together, dissociate the mutagenicity of MDA from its ability to form interstrand cross-links with DNA.

Template activity of calf thymus DNA modified by a dihydrodiol epoxide derivative of benzo[a]pyrene

The purpose of the present study was to determine the effects of covalent binding to DNA of a reactive derivative of benzo[a]pyrene on template activity during in vitro transcription with RNA polymerase. Calf thymus deoxyribonucleic acid, modified by reaction with (+/-)-7beta,8alpha-dihydroxy-9alpha, 10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, was transcribed with Escherichia coli DNA-dependent RNA polymerase. With increasing levels of modification, there was a progressive inhibition of transcription. The inhibition was much greater under conditions where continuous reinitiation of transcription occurred than under conditions where only one RNA chain was synthesized per initiation site. This suggested that the modified sites block the movement of polymerase along the template and prevent recycling of the enzyme. Consistent with this interpretation were analyses of RNA transcripts on sucrose density gradients which showed a progressive decrease in average RNA chain length as the extent of template modification increased. In contrast to the inhibitory effect on chain elongation, evidence was obtained that the modified DNA had an increase in the number of initiation sites for transcription. These results are consistent with separate physical studies indicating that modification of DNA by this benzo[a]pyrene derivative can induce small localized regions of denaturation.

Studies on liver chromatin RNA from rats treated with alkylating agents

We have examined firstly some properties of rat liver chromatin RNA and nuclear sap RNA and secondly the incorporation of [3H]orotic acid into the RNA in vivo in control rats and in rats treated with the alkylating agents, N,N-dimethylnitrosamine or methyl methane sulphonate. Half or more of the nuclear RNA is associated with the chromatin and consists mainly of two species: one is labelled and probably comprises "nascent" RNA, and the other is unlabelled and of lower molecular weight. Neither species is attributable to cytoplasmic contamination. Studies with added polylysine with RNAase A and with DNAase I suggest that both species are ironically bound to protein and that the labelled species is not associated with the part of the chromatin DNA most readily degraded by DNAase I. After dimethylnitrosamine treatment, the amount of unlabelled RNA remains constant but the amount of labelled RNA increases after a low dose, and decreases after a high dose. After methyl methane sulphonate treatment, no change occurs in either species. These results can be explained by changes in extent of association of the DNA and protein within the chromatin complex.