Amino acid sequences are shown as single letter codes. The numbering at the bottom is the amino acid sequence from T. thermophilus RRF. Identical residues are shadowed in black and similar residues are boxed. The corresponding secondary-structure elements of EcoRRF are indicated at the top. Vertical arrows indicate the points where the two domains were exchanged (G30 and R31, P103 and P104 for EcoRRF; G29 and R30, P102 and E103 for TteRRF respectively). (B) Schematic drawings of the structures of EcoRRF, TteRRF, TteDI/EcoDII, and EcoDI/TteDII.