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-tRNA(Sec), which is subsequently converted by selenocysteine synthase to Sec-tRNA(Sec). Escherichia coli SerRS serylates tRNA(Sec) 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-tRNA(Sec) synthesis. They display 10-, 8-, and 4-fold increased k(cat)/K-M values compared to wild-type SerRS using synthetic tRNA(Sec) species as substrates. The enzyme variants also facilitate in vivo read-through of a UAG codon in the position of the critical serine(146) of chloramphenicol acetyltransferase. These results indicate that the naturally evolved SerRS is capable of further evolution for increased recognition of a specific tRNA isoacceptor.