Codon Reading by tRNAAla with Modified Uridine in the Wobble Position

SummarytRNAs reading four-codon families often have a modified uridine, cmo5U34, at the wobble position of the anticodon. Here, we examine the effects on the decoding mechanism of a cmo5U modification in tRNA1BAla, anticodon C36G35cmo5U34. tRNA1BAla reads its cognate codons in a manner that is very similar to that of tRNAPhe. As Ala codons are GC rich and Phe codons AU rich, this similarity suggests a uniform decoding mechanism that is independent of the GC content of the codon-anticodon duplex or the identity of the tRNA. The presence of cmo5U at the wobble position of tRNA1BAla permits fairly efficient reading of non-Watson-Crick and nonwobble bases in the third codon position, e.g., the GCC codon. The ribosome accepts the C-cmo5U pair as an almost-correct base pair, unlike third-position mismatches, which lead to the incorporation of incorrect amino acids and are efficiently rejected.