Role of RNA Polymerase, ρ Factor, and Ribosomes in Transcription Termination

Abstract

INTRODUCTION
Efficient transcription of the Escherichia coli genome depends on the ability of the cell to regulate the type and amount of mRNA produced from each of its 3000 to 4000 genes. This not only requires the information to specify precise starting points for transcription, but stopping points as well. The operon, as the functional polycistronic unit of gene expression, may in fact be defined by the occurrence of an efficient termination site at its distal end. This simple housekeeping function prevents transcriptional readthrough by RNA polymerase into adjoining regions of the genome. In addition, however, a termination site can be utilized within a regulatory region to alter gene expression by controlling the ability of RNA polymerase to transcribe beyond that site.

Thus, sites that specify termination may have several purposes in the cell. (1) As efficient punctuation signals, they permit the differential regulation of adjacent gene clusters. (2) As modulating elements (e.g., in attenuation), they permit differential control of expression within operons. (3) As conditional abortive elements (e.g., in mutational polarity), they prevent wasteful depletion of cellular metabolites. (4) As barriers to elongation, they minimize sequestering of important enzymes (e.g., RNA polymerase or other RNA-binding macromolecules) that might be engaging in unnecessary transcription or binding to excess unusable products.

It has been generally expected that all termination sites, regardless of their function in regulation, would contain certain common structural features required for the termination response of RNA polymerase at that site. The primary structures (sequences) of a large number...