Specific Inhibition of Nuclear RNA Polymerase II by α-Amanitin Author(s):

a-Amanitin, a toxic substance from the mushroom Amanita phalloides, is a potent inhibitor of DNA-dependent RNA polymerase II (the nucleoplasmic form) from sea urchin, rat liver, and calf thymus. This compound exerts no effect on the activity of polymerase I (nucleolar form) or polymerase III (also nucleoplasmic). The inhibition is due to a specific interaction with polymerase II or with a complex of DNA and polymerase II. We have described the physical separation of three distinct DNA-dependent RNA polymerases from sea urchin embryos and two polymerases from rat liver (1, 2). Polymerase I resides in the nucleolus, and polymerases II and III reside in the nucleoplasm (2). The enzymes exhibit distinctive variations in activity with magnesium, manganese, and salt concentration changes. It is inferred that the polymerases have different functions, but for definitive investigations on this point specific inhibitors would prove highly useful. We have tested the effect of a-amanitin [the toxic bicyclic octapeptide from the mushroom Amanita phalloides (3)] on the various nuclear polymerases. Administration of this compound to mice results in fragmentation of liver nucleoli (4) and an overall decrease in nuclear RNA content (5). Furthermore, in the presence of Mn++ and high concentrations of salt, a-amanitin partially inhibits the polymerase activity of mouse liver nuclei (6) and of crude nuclear preparations (7). We now report that a-amanitin specifically inhibits polymerase II from the nuclei of several organisms although polymerase I and III activities are not affected. We also provide substantial