Fig. 1.

Structures and sequences of the boxB RNA, the WT N peptide, and their complex (14, 15). The RNA backbone and bases are gray, and A7, A8, and A9 are labeled. The WT N peptide binds as a bent α-helix, separating the structure into N- and C-terminal helical domains. The peptide chain is represented as red ribbon with the side chains of Lys-14, Gln-15, and Trp-18 shown. The chemical structures of amino acids incorporated into positions 14 and 15 of N peptides are shown at Right. Arrows indicate the protons that are either observed to be involved in H-bonding to carbonyl of Arg-8 in WT complex of box/N (14, 15) or boxB/Nun (28) or hypothesized to participate in similar interactions in the mutant complexes (see text).