Structural and functional studies on mammalian nuclear DNA-dependent RNA polymerases.

Abstract

Although DNA-dependent RNA polymerase was first identified in rat liver nuclei (1), this enzyme has only recently been solubilized and purified from eukaryotic cells, due to difficulties in separating the enzyme from nuclear chromatin, the relatively small amounts of enzyme in animal cells, and its marked instability when separated from chromatin. One of the most interesting findings was the existence of multiple nuclear DNA-dependent RNA polymerases [for references, see (2)], which suggested that gene expression in animal cells could be regulated, at least in part, by distinct RNA polymerases with different template specificities. Three lines of evidence supported the existence of multiple RNA polymerases. Several peaks of enzyme activity were obtained by chromatography on substituted cellulose columns (3,4,7), and these activities appeared to have distinctive intranuclear localizations (4–6). Furthermore, two classes of enzyme were distinguished, according to the inhibitory effect of α-amanitin (7,15), a toxin of the toadstool Amanita phalloides. More recently, complete purification and structural analysis of some of the RNA polymerase activities have firmly established the multiplicity of RNA polymerases in animal tissues (8,9). This led us to propose a terminology for animal DNA-dependent RNA polymerase based both on the inhibitory effect of amanitin and the subunit structure of the enzyme (8) (Table I).