Studies on the mechanism of the stimulation of polymerase II-catalyzed RNA synthesis by mercury compounds.

The specific stimulation of alpha-amanitin-sensitive RNA synthesis in isolated nuclei by methyl mercury (Frenkel, G. D., and Randles, K. (1982) J. Biol. Chem. 257, 6275-6279) has been further investigated. Using the method of alkaline hydrolysis/uridine analysis to determine the number of RNA chains growing in vitro, it was found that the stimulation could not be accounted for by an increase in the number of growing chains. The stimulatory effect of heparin (Coupar, B. E. H., and Chesterton, C. J. (1977) Eur. J. Biochem. 79, 525-533), was found to be additive with that of methyl mercury at saturating concentrations of the latter. Various detergents were found to affect RNA synthesis per se and to modify the stimulatory effect of methyl mercury, suggesting that the stimulation by methyl mercury requires a degree of structural integrity of some nuclear components. The ability of a number of other mercury compounds to stimulate RNA synthesis was investigated. None of the inorganic compounds examined, i.e. HgCl2, HgSO4, and Hg(ClO4)2, stimulated synthesis. Among the alkyl organic compounds tested in addition to methyl mercury, ethyl mercury also stimulated RNA synthesis, but dimethylmercury did not. Among the aryl compounds tested, phenylmercury did not stimulate synthesis whereas p-hydroxymercuribenzoate and p-hydroxymercuribenzenesulfonate did. N-Ethylmaleimide, a nonmercurous sulfhydryl reagent, was found to have only weak ability to stimulate RNA synthesis, compared to a comparable mercury-containing sulfhydryl reagent, p-hydroxymercuribenzoate. The stimulatory effect of the latter was, however, effectively competed out by the former, indicating that sulfhydryl binding is necessary for the stimulation but not sufficient. This conclusion was reinforced by experiments which utilized a model system to measure the ability of various mercury compounds to compete with N-ethylmaleimide in binding to cysteine. The results showed that even compounds such as phenylmercury and the inorganic mercurials, which are unable to stimulate RNA synthesis, are able to bind to a sulfhydryl group.