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.