Specific stimulation of alpha-amanitin-sensitive RNA synthesis in isolated HeLa nuclei by methyl mercury

Methyl mercury has previously been shown to inhibit DNA and RNA synthesis in intact HeLa cells (Gruenwedel, D. W., and Cruikshank, M. K. (1979) Biochem. Pharmacol. 28, 651-655). Its effect on nucleic acid synthesis in vitro in isolated nuclei has now been investigated. DNA synthesis in nuclei was inhibited by methyl mercury; 50% inhibition was obtained with a concentration of 8 microM, compared to approximately 30 microM which is required in vivo. In contrast, RNA synthesis in HeLa nuclei was stimulated by methyl mercury. This stimulation is specific for alpha-amanitin-sensitive RNA synthesis (catalyzed by RNA polymerase II); in contrast, alpha-amanitin-resistant synthesis (catalyzed by polymerases I and III) was inhibited. Transcription of calf thymus DNA by solubilized RNA polymerases was inhibited by ethyl mercury. However, if the enzymes were preincubated with the DNA prior to addition of the methyl mercury, subsequent synthesis by RNA polymerase II was stimulated, whereas the inhibition of the other enzymes was unaffected. The most likely explanation for these results is that methyl mercury specifically stimulates chain elongation by RNA polymerase II whereas it inhibits elongation by polymerases I and III, and initiation by all three enzymes.