Evidence for class-specific discrimination of a semiconserved base pair by tRNA synthetases.

Aminoacyl-tRNA synthetases have been divided into two classes based on the existence of two structurally distinct active sites. To date, few class-specific tRNA recognition features have been elucidated. High-resolution X-ray structures of representative class I and class II synthetases complexed to cognate tRNA substrates have been solved. In these structures, the class I enzyme approaches the end of the tRNA acceptor stem from the minor-groove side, while the class II synthetase approaches its cognate tRNA from the major-groove side. This distinction is reflected in the different initial sites (2'- or 3'-OH) of amino acid attachment. The role that the semiconserved G1.C72 terminal base pair plays in the aminoacylation of Escherichia coli tRNAs is probed in this in vitro study. We show here that class II alanyl-, prolyl-, and histidyl-tRNA synthetases are sensitive to changes at position 1 x 72. Previous work on class I synthetases and new data presented here with the valine-specific enzyme indicate that class I enzymes show little sensitivity to replacements of G1.C72. This work provides new evidence for class-specific differences in tRNA acceptor stem interactions that appear to be reflected not only in the initial site of aminoacylation but also in the mode of synthetase interaction with the semiconserved G1.C72 base pair proximal to the amino acid attachment site.