ABSTRACT
RNA transcription of mononegavirales decreases gradually from the 3' leader promoter toward the 5' end of the genome, due to a decay in polymerase processivity. In the respiratory syncytial virus and metapneumovirus, the M2-1 protein ensures transcription anti-termination. Despite being a homotetramer, respiratory syncytial virus M2-1 binds two molecules of RNA of 13mer or longer per tetramer, and temperature-sensitive secondary structure in the RNA ligand is unfolded by stoichiometric interaction with M2-1. Fine quantitative analysis shows positive cooperativity, indicative of conformational asymmetry in the tetramer. RNA binds to M2-1 through a fast bimolecular association followed by slow rearrangements corresponding to an induced-fit mechanism, providing a sequential description of the time events of cooperativity. The first binding event of half of the RNA molecule to one of the sites increases the affinity of the second binding event on the adjacent contacting protomer by 15-fold, product of increased effective concentration caused by the entropic link. This mechanism allows for high-affinity binding with an otherwise relaxed sequence specificity, and instead suggests a yet undefined structural recognition signature in the RNA for modulating gene transcription. This work provides a basis for an essential event for understanding transcription antitermination in pneumoviruses and its counterpart Ebola virus VP30.