Figure 2.

mRNA structures that affect MHC‐I mediated antigen presentation. (a) RNA G‐quadruplexes within gammaherpes viral ORFs negatively impact antigen presentation. G‐quadruplex structures are formed by guanine‐rich sequences and are stabilized by the stacking of guanine‐tetrads that are formed by the coplanar arrangement of four guanines interacting by Hoogsteen hydrogen‐bonding and stabilized by monovalent cations (usually K⁺). (b) Cloverleaf structures identified in the 5'‐untranslated regions of poliovirus. Cloverleaf structures in enteroviruses control the replication and translation of the viral genomes through the formation of ribonucleoprotein complexes. (c) HIV‐1 programmed ribosomal frameshift signal secondary structure. The formation of the stem‐loop structure downstream of a slippery sequence leads to the translocating ribosome to stall and induce a ‐1 frameshift that insures the synthesis of pol and gag which are encoded by the same ORF.