Myogenic differentiation of L6 rat myoblasts: evidence for pleiotropic effects on myogenesis by RNA polymerase II mutations to alpha-amanitin resistance

Mutations in the alpha-amanitin conserved domain of the largest subunit of yeast RNA polymerase III affect pausing, RNA cleavage and transcriptional transitions

The alpha-amanitin domain or domain f of the largest subunit of RNA polymerases is one of the most conserved of these enzymes. We have found that the C-terminal part of domain f can be swapped between yeast RNA polymerase II and III. An extensive mutagenesis of domain f of C160, the largest subunit of RNA polymerase III, was carried out to better define its role and understand the mechanism through which C160 participates in transcription. One mutant enzyme, C160-270, showed much reduced transcription of a non-specific template at low DNA concentrations. Abortive synthesis of trinucleotides in a dinucleotide-primed reaction proceeded at roughly wild-type levels, indicating that the mutation did not affect the formation of the first phosphodiester bond, but rather the transition from abortive initiation to processive elongation. In specific transcription assays, on the SUP4 tRNA gene, pausing was extended but the rate of RNA elongation between pause sites was not affected. Finally, the rate of cleavage of nascent RNA transcripts by halted mutant RNA polymerase was increased approximately 10-fold. We propose that the domain f mutation affects the transition between two transcriptional modes, one being adopted during abortive transcription and at pause sites, the other during elongation between pause sites.

Sensitivity of Carrot Cell Cultures and RNA Polymerase II to Amatoxins : Evidence for the Inactivation of 6'-Hydroxyamatoxins

Protoplast and cell suspension cultures of Daucus carota L. were evaluated for their sensitivity toward the three amatoxin derivatives, alpha-amanitin, 6'-deoxy-alpha-amanitin, and 6'-O-methyl-alpha-amanitin using inhibition of DNA synthesis to measure cell viability. Protoplasts appeared approximately 10-fold more refractory than suspension cells and alpha-amanitin was much less effective than the other two amatoxins, even though K(i) values for isolated RNA polymerase II were similar (4-5 nanomolar). Additional studies evaluating the recoveries of all three amatoxins from cell suspension supernates indicate one basis for these differences to be the selective degradation of alpha-amanitin. A mechanism involving the activation of the hydroxyindole moiety of the alpha-amanitin is thus invoked to explain these differences and we postulate the involvement of plant oxidases in this role.