Effects of mutations at position 36 of tRNA(Glu) on missense and nonsense suppression in Escherichia coli.

Mutations in the anticodon of tRNA(Glu) (UUC) were isolated or constructed and characterized for their ability to suppress cognate nonsense or missense mutations in vivo. The C36-to-A36 transversion mutation was isolated as an ochre and an amber suppressor, while the G36 transversion was selected as a CAG missense suppressor. tRNA(Glu) suppressors of an AAG missense mutation could not be isolated, and a U36 transition mutation introduced into tRNA(Glu) in vitro conferred no suppressor phenotype. Over-expression of glutamyl-tRNA synthetase did not increase the activity of the U36 mutant tRNA(Glu), suggesting a defect at the level of translation rather than at the level of synthetase recognition.