Designing seryl-tRNA synthetase for improved serylation of selenocysteine tRNAs.

Selenocysteine (Sec) lacks a cognate aminoacyl-tRNA synthetase. Instead, seryl-tRNA synthetase (SerRS) produces Ser-tRNAS ec , which is subsequently converted by selenocysteine synthase to Sec-tRNAS ec . Escherichia coli SerRS serylates tRNAS ec poorly; this may hinder efficient production of designer selenoproteins in vivo. Guided by structural modelling and selection for chloramphenicol acetyltransferase activity, we evolved three SerRS variants capable of improved Ser-tRNAS ec synthesis. They display 10-, 8-, and 4-fold increased kcat /KM values compared to wild-type SerRS using synthetic tRNAS ec species as substrates. The enzyme variants also facilitate in vivo read-through of a UAG codon in the position of the critical serine146 of chloramphenicol acetyltransferase. These results indicate that the naturally evolved SerRS is capable of further evolution for increased recognition of a specific tRNA isoacceptor.