Studies on the Mechanism of Action of the Gal Repressor

The mechanism of action of Escherichia coli gal repressor on in vitro gal messenger RNA synthesis has been studied. The purified transcription system includes RNA polymerase holoenzyme, cyclic adenosine 3′:5′-monophosphate (cyclic AMP), cyclic AMP receptor protein (CRP), and a λpgal DNA template. We find that gal repressor acts in a manner competitive with RNA polymerase plus CRP. A preinitiation complex formed by λpgal DNA, RNA polymerase, CRP, and cyclic AMP is resistant to inhibition by gal repressor. Conversely, incubation of λpgal DNA with gal repressor prior to the addition of RNA polymerase, CRP, and cyclic AMP prevents the formation of a preinitiation complex. We conclude that the gal operator and promoter regions overlap functionally, and we consider a variety of models to explain this effect.