A new RNA structural motif consisting of two double helices closely packed via minor grooves is found in many places in the ribosome structure. The packing requires that a GU base pair in one helix be packed against a Watson–Crick pair in the other helix. Two such motifs mediate the interaction of the P- and E-tRNA with the large ribosomal subunit. Analysis of the particular positions of these two motifs in view of the available data on occupancy of tRNA-binding sites and structural changes in the ribosome during the elongation cycle suggests a distinct role for each motif in tRNA translocation.