Noncoding, structured 5’- untranslated regions (5’-UTRs) of messenger RNAs (mRNAs) control translation efficiency by forming structures that can either recruit or repel the ribosome. Here we exploit a bacterial, preQ1-sensing translational riboswitch to probe how binding of a small ligand controls binding of the bacterial ribosome to the Shine-Dalgarno (SD) sequence. Combining single-molecule fluorescence microscopy with mutational analyses, we find that the stability of 30S ribosomal subunit binding is inversely correlated with the free energy needed to unfold the 5’-UTR during mRNA accommodation from the standby site to the binding cleft. Ligand binding stabilizes 5’-UTR structure to both antagonize 30S recruitment and accelerate 30S dissociation. Depletion of small ribosomal subunit protein S1, known to resolve structured 5’-UTRs, further increases the energetic penalty for mRNA accommodation. The resulting model of rapid standby site exploration followed by gated non-equilibrium unfolding of the 5’-UTR during accommodation provides a mechanistic understanding of translation efficiency.