25 Prokaryotic Transcriptional Enhancers

Abstract

OVERVIEW
All known prokaryotic enhancer-binding proteins activate transcription by alternative holoenzyme forms of RNA polymerase, rather than σ⁷⁰ holoenzyme. The enhancer-binding proteins activate transcription by stimulating a change in configuration of RNA polymerase at a promoter—specifically, the isomerization of a closed recognition complex to an open complex. To catalyze isomerization, the enhancer-binding proteins must hydrolyze ATP, making them the only transcriptional activators known to have a required enzymatic activity.

On the basis of their cognate RNA polymerases, prokaryotic enhancer-binding proteins can be divided into two classes. One class activates transcription by σ⁵⁴ holoenzyme and has been shown to contact this polymerase by means of a DNA loop. At some, but not all, σ⁵⁴- dependent promoters, loop formation is assisted by a DNA-bending protein, IHF. The best-studied activator of σ⁵⁴ holoenzyme is the NTRC protein; the ability of NTRC to activate transcription is regulated by phosphorylation, which controls its ATPase activity. The other class of prokaryotic enhancer-binding proteins, which is represented by a single member, activates transcription by a holoenzyme that contains both a sigma factor and an adapter protein encoded by phage T4. The T4 enhancer-binding protein is unusual in that it probably does not control the function of its cognate polymerase by a simple looping mechanism.

INTRODUCTION
Transcriptional enhancers play a critical role in the differentiation and development of eukaryotic organisms and in control of their metabolism (Gehring 1987; Maniatis et al. 1987; Levine and Hoey 1988; Muller et al. 1988). Hence the mechanism(s) by which enhancers communicate...