1 Yeast Nuclear RNA Polymerases

Abstract

INTRODUCTION
All eukaryotes analyzed so far contain three distinct nuclear DNA-dependent RNA polymerases, each transcribing a different set of genes. In the past decade, studies with reconstituted RNA-polymerase-dependent systems, combined with genetic evidence obtained mainly in yeast, have established that RNA polymerase I (or A) synthesizes ribosomal RNA precursors, RNA polymerase II (or B) transcribes protein-encoding genes, and RNA polymerase III (or C) produces 5S rRNA and tRNAs. Most of the small nuclear RNAs (snRNAs) are transcribed by enzyme II or, in some cases, by enzyme III. Each enzyme is directed for promoter selection and correct initiation by a set of general and gene-specific transcription factors. The characterization of yeast RNA polymerases, transcription factors, and regulatory proteins has greatly contributed to our overall understanding of eukaryotic transcription.

In this chapter, we provide an overview of the molecular structure of the three yeast nuclear RNA polymerases as derived from biochemical and immunological studies, present recent progress made in characterizing the genes encoding the polymerase subunits, and discuss the specific in vitro transcription systems now available for all three yeast RNA polymerases, with special emphasis on the RNA polymerase I and III systems. The regulatory aspects of RNA-polymerase-II-dependent transcription are presented by Guarente (this volume).

II. MOLECULAR STRUCTURE OF NUCLEAR RNA POLYMERASES: AN OVERVIEW
A. Enzyme Purification
Yeast RNA polymerases are large multisubunit proteins ranging in size from 500 kD to 600 kD (Sentenac and Hall 1982; Sentenac 1985 and references therein). Their general physical properties and affinity for nucleic acids or...