Specific action of alpha-amanitin on mammalian RNA polymerase protein

Reticulocyte RNA-dependent RNA polymerase

A cytoplasmic, microsomal bound RNA-dependent RNA polymerase has been purified 2500-fold from rabbit reticulocyte lysates. The synthesis of RNA with the purified enzyme is absolutely dependent on the addition of an RNA template. The best template is hemoglobin messenger RNA, while bacteriophage RNA and poly(A,G) are less active, and DNA is completely inactive as a template. With poly(A,G) as a template, only UTP and CTP are incorporated into polynucleotide chains, indicating that the RNA polymerase is an RNA replicase and not a terminal transferase. With messenger RNA as a template, all four ribonucleoside triphosphates are required for maximal activity. The RNA-dependent RNA polymerase reaction is extremely sensitive to low concentrations of heme, rifamycin AF/013, and ribonuclease and resistant to actinomycin D and DNase. The discovery of RNA-directed RNA synthesis in reticulocytes offers an additional site for control of gene expression in mammalian cells and provides a possible mechanism for amplification of the expression of specific genes.

Regulation of RNA synthesis in fibroblasts during transition from resting to growing state

Addition of serum, containing fibroblast growth factors, to a culture of resting 3T6 cells stimulates a transition to the growing state. Studies of ghost monolayers prepared with the aid of detergent at intervals after stimulation showed an increase in the rate of ribosomal RNA synthesis within 10 min. The rate continued to increase for many hours and reached a level 2.5- to 3.5-fold higher by the time DNA synthesis began. The increasing rate of ribosomal RNA synthesis appeared independent of an increase in the number of ribosomal genes, since it was not affected by prevention of DNA synthesis with cytosine arabinoside. In contrast to ribosomal RNA, the overall rate of transscription of heterogeneous nuclear RNA was not directly affected by serum growth factors and does not appear to be regulated during the transition from resting to growing state. It seems, instead, to be fixed in relation to the amount of template, for it increases proportionally to DNA content.

Mammalian cells with altered forms of RNA polymerase II

Mutants of Chinese hamster ovary cells that are resistant to alpha-amanitin can be isolated. At least some of these mutants contain an altered form of DNA-dependent RNA polymerase II, as indicated by its resistance to alpha-amanitin. These results indicate that mutation to alpha-amanitin resistance involves a change of a structural gene.

Solubilized DNA-dependent RNA polymerase from human placenta: A Mn(2+)-dependent enzyme

The 100-fold purified RNA polymerase activity from human placenta is completely dependent upon added DNA. The enzyme is most active at 3 mM Mn(2+) in the presence of 100 mM (NH(4))(2)SO(4). Denatured DNA is a better template than native DNA. alpha-Amanitin completely inhibits the incorporation of 3H-UMP, while rifampicin has no influence upon the enzymatic activity.