Nat. Struct. Biol. 10, 781–788 (2003).
Two mistakes are found in the final version of this manuscript. The first is on page 784 (second column, line 19 from the bottom); the correct sentence should read: “... in which the base U33 forms hydrogen bonds with the main chain amide and carbonyl groups of Gly263....” The second is on page 786, where Figure 6 was incorrectly referred to six times in the Results section. The correct figure described on this page is Figure 7. We apologize for any inconvenience this may have caused.