(A) Hydrogen bonding and steric interactions between the first two nucleotides of the UAA stop codon with the reading head of RF1 in T. thermophilus(from PDB entry 3D5A ). (B) Molecular model of the reading head conformation in the mtRF1. Residues at positions interacting with the stop codon in panel A are shown. (C) Stabilizing interaction between A-1493 of the ribosomal decoding centre (shown in blue) and the switch loop (shown in green) of release factor RF1 in T. thermophilus (from PDB entry 3MR8 ). (D) Stabilizing interaction between the arginine of the RT-insert and the switch loop of mtRF1 when the A-site is empty. Coordinates of the ribosomal decoding centre (shown in blue) are taken from PDB entry 1IBM . All numbering according to the T. thermophilus RF1 sequence.