Structure-specific tRNA determinants for editing a mischarged amino acid.

Alanyl-tRNA synthetase efficiently aminoacylates tRNAAla and an RNA minihelix that comprises just one domain of the two-domain L-shaped tRNA structure. It also clears mischarged tRNAAla using a specialized domain in its C-terminal half. In contrast to full-length tRNAAla, minihelixAla was robustly mischarged and could not be edited. Addition in trans of the missing anticodon-containing domain did not activate editing of mischarged minihelixAla. To understand these differences between minihelixAla and tRNAAla, several chimeric full tRNAs were constructed. These had the acceptor stem of a non-cognate tRNA replaced with the stem of tRNAAla. The chimeric tRNAs collectively introduced multiple sequence changes in all parts but the acceptor stem. However, although the acceptor stem in isolation (as the minihelix) lacked determinants for editing, alanyl-tRNA synthetase effectively cleared a mischarged amino acid from each chimeric tRNA. Thus, a covalently continuous two-domain structure per se, not sequence, is a major determinant for clearance of errors of aminoacylation by alanyl-tRNA synthetase. Because errors of aminoacylation are known to be deleterious to cell growth, structure-specific determinants constitute a powerful selective pressure to retain the format of the two-domain L-shaped tRNA.