Folding behavior of a T-shaped, ribosome-binding translation enhancer implicated in a wide-spread conformational switch

Turnip crinkle virus contains a T-shaped, ribosome-binding, translation enhancer (TSS) in its 3’UTR that serves as a hub for interactions throughout the region. The viral RNA-dependent RNA polymerase (RdRp) causes the TSS/surrounding region to undergo a conformational shift postulated to inhibit translation. Using optical tweezers (OT) and steered molecular dynamic simulations (SMD), we found that the unusual stability of pseudoknotted element H4a/Ψ(3) required five upstream adenylates, and H4a/Ψ(3) was necessary for cooperative association of two other hairpins (H5/H4b) in Mg(2+). SMD recapitulated the TSS unfolding order in the absence of Mg(2+), showed dependence of the resistance to pulling on the 3D orientation and gave structural insights into the measured contour lengths of the TSS structure elements. Adenylate mutations eliminated one-site RdRp binding to the 3’UTR, suggesting that RdRp binding to the adenylates disrupts H4a/Ψ(3), leading to loss of H5/H4b interaction and promoting a conformational switch interrupting translation and promoting replication. DOI: http://dx.doi.org/10.7554/eLife.22883.001