Probing RNA Structure and Dynamics in the HIV‐1 5′ UTR using Ensemble and Single Molecule Approaches

Abstract

One of the most conserved and highly structured regions of the HIV‐1 genome, the 356‐nt 5′UTR, acts as a scaffold for many protein and nucleic acid binding partners and regulates vital processes including initiation of reverse transcription, genomic RNA (gRNA) dimerization and packaging. Previous studies support the presence of at least two mutually‐exclusive 5′UTR conformational states (Huthoff and Berkhout, RNA, 7, 2001; Lu et al., Science 334 (2011), but whether the 5′UTR undergoes a conformational switch during the viral lifecycle and what regulates this switch are less clear. In‐gel selective 2′‐hydroxyl acylation followed by primer extension (SHAPE) probing studies of the wild‐type (WT) 5′UTR revealed a mixed population with structural differences between monomer and dimer, consistent with a structural switch (Kenyon, JC et al, Nucleic Acids Res 41, 2013). However, these studies did not address the dynamics of the conformational change or probe possible triggers. In this work, we used single‐molecule Förster resonance energy transfer (SM‐FRET) studies to probe 5′UTR (ΔTAR/polyA) conformational dynamics and the factors that modulate it. Intra‐molecular FRET assays showed that mutations in the dimerization initiation site (DIS) and U5 region of the monomeric RNA modulated the conformational equilibrium in the expected manner, with the ΔDIS construct shifted to a high‐FRET dimerization‐competent state. Increasing Mg2+ concentration promoted the dimer‐competent conformation in the WT 5′UTR. In 10 mM Mg2+, the WT construct occupies both the high‐FRET (39%) and low‐FRET states (61%). Individual FRET traces showed that of the 841 molecules examined under these conditions, 29% transitioned between FRET states during the ~150 sec observation window. Inter‐molecular FRET observed between an immobilized donor‐labeled 5′UTR construct and excess acceptor‐labeled 5′UTR was consistent with formation of a stable extended dimer. In‐gel FRET assays were also consistent with the formation of a stable dimer. Surprisingly, tRNA‐annealing to the primer binding site of an immobilized 5′UTR promoted the monomer conformation even in the presence of high Mg2+, as observed by both SM‐FRET and native PAGE. Overall, these data are consistent with high intrinsic conformational dynamics in the 5′UTR that can be regulated by factor binding.Support or Funding InformationNational Institutes of HealthThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.