Hydrolytic editing by a class II aminoacyl-tRNA synthetase

Editing reactions catalyzed by aminoacyl-tRNA synthetases are critical for accurate translation of the genetic code. To date, this activity, whereby m isactivated amino acids are hydrolyzed either before or after transfer to n oncognate tRNAs, has been characterized extensively only in the case of cla ss I synthetases. Class II synthetases have an active-site architecture tha t is completely distinct from that of class I. Thus, findings on editing by class I synthetases may not be applicable generally to class II enzymes. C lass II Escherichia coli proline-tRNA synthetase is shown here to misactiva te alanine and to hydrolyze the noncognate amino acid before transfer to tR NA(Pro). This enzyme also is capable of rapidly deacylating a mischarged Al a-tRNA(Pro) variant. A single cysteine residue (C443) that is located withi n the class II-specific motif 3 consensus sequence was shown previously to be dispensable for proline-tRNA synthetase aminoacylation activity. We show here that C443 is critical for the hydrolytic editing of Ala-tRNA(Pro) by this class II synthetase.