19 Transcriptional Activation by Escherichia coli CAP Protein

Abstract

OVERVIEW
The interaction of the cAMP-binding E. coli transcriptional regulatory protein CAP (or CRP) with DNA is relatively well understood at the structural level, yet major questions concerning its mechanism of action remain unresolved. CAP binds upstream of RNA polymerase at a variety of distances, activating some promoters and repressing others. We focus our attention on gene activation by CAP when bound 41 or 61 base pairs upstream of the transcription start site, as found in the gal and lac promoters, respectively. CAP binding at those positions can assist in the initial capture of RNA polymerase to form the open complex, or it can accelerate the rate of isomerization to the open complex, or it can do both. Two primary possibilities can be identified for the mechanism of action: CAP and RNA polymerase may interact through specific protein-protein contacts, or the two proteins may act cooperatively by sharing a preference for bent DNA. The latter model requires that polymerase contact DNA both upstream and downstream from the CAP-binding site in order to form a DNA loop that entraps CAP. We describe evidence favoring both mechanisms, and we suggest that both may contribute to CAP activation of transcription.

INTRODUCTION
The catabolite activator protein (CAP) or cAMP receptor protein (CRP) was discovered more than 20 years ago (Emmer et al. 1970; Zubay et al. 1970; Anderson et al. 1971), marking an important milestone in the continuing effort to understand the regulatory response of E. coli when the growth medium is switched between...