Molecular Structures of Eukaryotic Class III RNA Polymerases

Abstract

INTRODUCTION
DNA-dependent RNA polymerase III represents one of the three major classes of nuclear RNA polymerases in eukaryotes (Roeder and Rutter 1969). The class III enzymes have characteristic catalytic and chromatographic properties which distinguish them from the class I and II enzymes (Roeder and Rutter 1969; Adman, Schultz and Hall 1972; Roeder 1974a). In animal cells, these enzymes are also distinguished by their unique α-amanitin sensitivities (Schwartz et al. 1974; Sklar and Roeder 1975; Seifart and Benecke 1975; Weil and Blatti 1975). Although lower eukaryotes may contain only a single chromatographic form of RNA polymerase III (Roeder and Rutter 1969; Adman, Schultz and Hall 1972), multiple chromatographic forms have been described in a variety of mammalian cell types (Schwartz et al. 1974; Jaehning, Stewart and Roeder 1975; Weinmann et al. 1976; Sklar and Roeder 1975, 1976; Seifart and Benecke 1975).

In animal cells, a class III RNA polymerase(s) has been implicated in the synthesis of tRNA and 5S RNA species (Weinmann and Roeder 1974) and in the synthesis of low molecular weight viral RNAs (Weinmann, Raskas and Roeder 1974; Weinmann et al. 1976; Jaehning et al., this volume). Although RNA polymerase III usually accounts for only a small proportion of the total cellular RNA polymerase activity, it has been detected, using appropriate analytical methods, in all tissues examined (Schwartz et al. 1974). However, the cellular levels of RNA polymerase III vary among different cell types and in the same cell type under different physiological conditions (Roeder 1974b; Schwartz et al....