Eukaryotic Nuclear RNA Polymerases

Abstract

INTRODUCTION
The specific mechanisms involved in the regulation of transcription are largely unknown, but whatever their nature, they must directly or indirectly affect the activity of the DNA-dependent RNA polymerases involved in the transcription processes. To date, three structurally distinct classes of nuclear RNA polymerases have been identified and shown to have distinct transcriptive functions. Hence, the quantitative regulation of specific classes of genes may be effected by the cellular levels of distinct enzymes. Other mechanisms must be responsible for the differential transcription of a class of genes that are transcribed by a common enzyme. Considerable evidence now supports the hypothesis that chromatin structural modifications govern the accessibility of specific genes and hence their ability to be transcribed by an RNA polymerase. However, there may exist transcriptional controls, other than those imposed by chromatin structure or by RNA polymerase levels, that operate through specific RNA polymerases. Such controls might involve components that alter the interactions between a specific enzyme and a specific gene or genes. The primary interactions of such components could be either with the enzyme or with the template. Undoubtedly, the elucidation of the various regulatory mechanisms will require their reconstruction in cell-free systems from purified components. It seems probable, therefore, that it will become increasingly important to understand the structure, function and regulation of the eukaryotic RNA polymerases since these enzymes may be necessary for the specific transcriptional controls to be manifest in vitro as well as in vivo.

This review will attempt to summarize, in a...