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 app ears in RNA as a hairpin that destabilizes the elongation complex (EC). We demonstrate that negative and positive-termination factors control the effi ciency of termination primarily through a direct modulation of hairpin fold ing and, to a much lesser extent, by changing pausing at the point of termi nation. The mechanism controlling hairpin formation at the termination poin t relies on weak protein interactions with single-stranded RNA, which corre sponds to the upstream portion of the hairpin. Escherichia coli NusA protei n destabilizes these interactions and thus promotes hairpin folding and ter mination. Stabilization of these contacts by phage lambda N protein leads t o antitermination.