Bacterial RNA Polymerases: The Genetics and Control of Their Synthesis

INTRODUCTION The RNA polymerase (RPase) of Escherichia coli is an oligomer containing nonidentical subunits, α, β, β ′ and σ (Burgess; Zillig, Palm and Heil; both this volume). The purpose of this article is to show how the genes for the different subunits have been identified and located on the genetic map of Escherichia coli and to relate these discoveries to current parallel studies in other organisms. In addition, how the expression of these genes is controlled will be discussed; the purpose of the control mechanism is probably to relate the rate of RPase subunit synthesis to the immediate and long-term needs of the cell. ISOLATION AND CHARACTERIZATION OF RNA POLYMERASE MUTATIONS Identification of the β Gene ( rpoB ) For a number of reasons, the genetic analysis of the RNA polymerase of E. coli presents a complex problem. First, the enzyme is vital to the cell; this prevents the direct isolation of mutants with totally inactive sub-units and has made conditional-lethal mutants essential for the analysis. Second, it interacts with a variety of accessory factors to modulate the expression of different genes or groups of genes. Thus many conditional-lethal RPase mutants have no immediate and dramatic effect on gross RNA synthesis. Finally, since the constituent subunits have no known enzymatic activity of their own, the assignment of subunit mutations is unusually laborious. The first of these problems was circumvented (in the 1960’s) by the discovery of a new group of drugs—the rifamycins — that directly affect RNA polymerases (Hartmann et al. 1967;...