Formation of the RNA Polymerase–lac Promoter Open Complex

Abstract

INTRODUCTION
Expression of the lactose (lac) operon is known to result from and be regulated by the interaction of three specific proteins with a set of unique nucleotide sequences at the start of the operon: RNA polymerase with the promoter (P), the represser with the operator, and the positive activator protein, CAP, with the promoter. These interactions are diagrammatically presented in Figure 1. The most universal of these reactions is the RNA polymerase–promoter interaction. Recent studies have suggested that this interaction involves a series of steps, each of which may require a specific protein–nucleic acid recognition reaction (Chamberlin 1974).

RNA polymerase may form a relatively loose “closed complex” with a specific site on the DNA.

This closed complex could then be converted into an “open complex” in which there is a localized melting of the DNA.

The RNA polymerase can then initiate synthesis of the mRNA chain.

Sequence studies on lacP suggested that there may be a translocation of the RNA polymerase molecule during the steps just described, since mutations thought to alter the RNA polymerase–promoter interaction lie outside of the piece of DNA protected by RNA polymerase when present in the open complex (Dickson et al. 1975; J. Gralla, personal communication). This translocation might occur between steps 1 and 2 mentioned above or as a result of step 2.

In order to understand better how the different portions of the lac promoter sequence are involved in the various steps of the RNA polymerase–...