Equilibrium dissociation experiments of tmRNA on programmed ribosomes, measured by filter-binding assays. The maximum of binding that we could obtain, at the highest ribosome concentration, was set to 1, and the other values were measured on the 0 to 1 scale, expressed as “fraction bound”. ◆: 70S-tRNA stalled with a truncated mRNA leaving the A site empty, in the absence of SmpB. ▲: 70S-tRNA stalled with an mRNA bearing a sense codon at the A site, in the presence of SmpB. ■: 70S-tRNA stalled with a truncated mRNA leaving the A site empty, in the presence of SmpB.