An engineered class I transfer RNA with a class II tertiary fold

Structure-based engineering of the tertiary fold of Escherichia coli tRNA(2 )(Gln) has enabled conversion of this transfer RNA to a class II structure while retaining recognition properties of a class I glutamine tRNA. The new tRNA possesses the 20-nt variable stem-loop of Thermus thermophilus tRNA(S er). Enlargement of the D-loop appears essential to maintaining a stable te rtiary structure in this species, while rearrangement of a base triple in t he augmented D-stem is critical for efficient glutaminylation. These data p rovide new insight into structural determinants distinguishing the class I and class II tRNA folds, and demonstrate a marked sensitivity of glutaminyl -tRNA synthetase to alteration of tRNA tertiary structure.