ROLE OF TRANSFER-RNA MODIFICATION IN TRANSLATIONAL FIDELITY

In transfer RNA many different modified nucleosides are found, especially in the anticodon region. In this region, pseudouridine (ψ) is found in positions 38, 39 or 40 in a subset of tRNA species, 2-methylthio-6-hydroxyisopentenyladenosine (ms2io6A) is found in position 37 in tRNAs that read codons starting with U and 1-methylguanosine (m1G) is found in position 37 in tRNAs reading codons of the {A figure is presented} type. We have used the mutants hisT, miaA and miaB and trmD, which are deficient in the biosynthesis of ψ, ms2io6A, and m1G, respectively, to study the functional aspects of the respective modified nucleosides. We have shown: (1) Presence of ψ improved the cellular growth rate, the polypeptide step-time, and the efficiency of an amber suppressor, but did not appreciably sense the codon context. (2) Presence of ms2io6A improved the cellular growth rate, the polypeptide step-time and the efficiency of several amber suppressor tRNAs. It also had a profound effect on the codon context sensitivity of the tRNA. (3) Presence of m1G improved the cellular growth rate and the polypeptide steptime and also prevented the tRNA from shifting the reading frame. Thus, these three modified nucleosides present in the anticodon region have apparently different functions. © 1990.