RNA tertiary interactions in the large ribosomal subunit: The A-minor motif

Analysis of the 2.4-Angstrom resolution crystal structure of the large ribo somal subunit from Haloarcula marismortui reveals the existence of an abund ant and ubiquitous structural motif that stabilizes RNA tertiary and quater nary structures. This motif is termed the A-minor motif, because it involve s the insertion of the smooth, minor groove edges of adenines into the mino r groove of neighboring helices, preferentially at C-G base pairs, where th ey form hydrogen bonds with one or both of the 2' OHs of those pairs. A-min or motifs stabilize contacts between RNA helices, interactions between loop s and helices, and the conformations of junctions and tight turns. The inte ractions between the 3' terminal adenine of tRNAs bound in either the A sit e or the P site with 23S rRNA are examples of functionally significant A-mi nor interactions. The A-minor motif is by far the most abundant tertiary st ructure interaction in the large ribosomal subunit; 186 adenines in 23S and 5S rRNA participate, 68 of which are conserved. It may prove to be the uni versally most important long-range interaction in large RNA structures.