Control of intrinsic transcription termination by N and NusA: the basic mechanisms

Intrinsic transcription termination plays a crucial role in regulating gene expression in prokaryotes. After a short pause, the termination signal appears in RNA as a hairpin that destabilizes the elongation complex (EC). We demonstrate that negative and positive termination factors control the efficiency of termination primarily through a direct modulation of hairpin folding and, to a much lesser extent, by changing pausing at the point of termination. The mechanism controlling hairpin formation at the termination point relies on weak protein interactions with single-stranded RNA, which corresponds to the upstream portion of the hairpin. Escherichia coli NusA protein destabilizes these interactions and thus promotes hairpin folding and termination. Stabilization of these contacts by phage lambda N protein leads to antitermination.