Converting Escherichia coli RNA Polymerase into an Enhancer-Responsive Enzyme: Role of an NH 2 -Terminal Leucine Patch in σ 54

Abstract

The protein σ⁵⁴ associates with Escherichia coli core RNA polymerase to form a holoenzyme that binds promoters but is inactive in the absence of enhancer activation. Here, mutants of σ⁵⁴ enabled polymerases to transcribe without enhancer protein and adenosine triphosphate. The mutations are in leucines within the NH₂-terminal glutamine-rich domain of σ⁵⁴. Multiple leucine substitutions mimicked the effect of enhancer protein, which suggests that the enhancer protein functions to disrupt a leucine patch. The results indicate that σ⁵⁴ acts both as an inhibitor of polymerase activity and as a receptor that interacts with enhancer protein to overcome this inhibition, and that these two activities jointly confer enhancer responsiveness.