The nature of the nucleotide at the 5′ side of the tRNA Su9 anticodon affects UGA suppression in Escherichia coli

In Escherichia coli a UGA codon can be efficiently suppressed by a suppressor tRNA^Trp called Su9. Here, we show that the level of UGA suppression is determined by the nature of the nucleotide at the 5′ side of the anticodon of the suppressor (position 33). UGA suppression occurs when a pyrimidine residue is located in position 33 of the tRNA, and suppression is more efficient with a U than with a C in this position. On the other hand, when a purine residue is located at this position UGA suppression is extremely low. These results show that in the case of tRNA Su9, the UGA codon context effect does not require base pairing between the nucleotide at the 3′ side of the codon and the 5′ side of the anticodon.