Equilibrium and non-equilibrium structure in the 5' UTR of MS2 RNA responsible for the transient translation of the A-protein gene. In panel C the folding intermediate is shown that allows translation. In panel B the final structure is depicted that is translationally inactive. Both structures are phylogenetically conserved in the E. coli leviviruses (95). LDI: long-distance interaction, SD: Shine-Dalgarno sequence, UCS: upstream complementary sequence. For convenience the UCS nts are shaded. For simplicity, potential structure involving the start codon and surrounding has not been drawn.