Yeast RNA Polymerases

Abstract

INTRODUCTION
The presence of multiple forms of RNA polymerases has been recognized in all eukaryotic cells, including plant, insect, fungi and yeast cells (Chambon 1975). A basic, generally accepted assumption is that these related enzymes are specifically synthesizing the various classes of RNA (Zylber and Penman 1971; Reeder and Roeder 1972; Weinmann and Roeder 1974; Chambon 1975). In animal cells, the α-amanitin-sensitive class B (or II) RNA polymerase is believed to synthesize precursor messenger RNA; the moderately sensitive class C (or III) enzyme is responsible for the synthesis of 5S and tRNA; whereas class A (or I) enzyme, which is insensitive to high concentrations of the toxic peptide, probably makes ribosomal RNA. Although it is still premature to extend this conclusion to lower eukaryotic cells, such as yeast cells, the striking similarity between enzymes isolated from various types of cells strongly supports this general hypothesis. One important problem, then, is to understand what determines, on a structural basis, the different specificities of the three classes of RNA polymerase.

Additional complexity arises from the presence of subclasses of RNA polymerases. For instance, RNA polymerase B can be resolved into two or three forms of enzyme, B0 (II₀), BI (II_A) and BII (II_B), which differ only in the molecular weight of their largest subunit (Chambon 1975; Link and Richter 1975; Schwartz and Roeder 1975). Two forms of RNA polymerase C can also be separated (Schwartz et al. 1974). The in vivo significance of this multiplicity is unclear. Therefore the origin and possible...