The modification of the wobble base of tRNAGlu modulates the translation rate of glutamic acid codons in vivo

In Escherichia coli, uridine in the wobble position of tRNA(Glu) and tRNA(Lys) is modified to mnm(5)s(2)U34. This modification is believed to restrict the base-pairing capability, i.e. to prevent misreading of near-cognate codons and reduce the efficiency of cognate codon reading, especially of codons ending in G. We have determined the influence of the 5-methylaminomethyl and the 2-thio modifications of mnm(5)s(2)U34 in tRNA(Glu) on the translation rate of the glutamate codons GAA and GAG in vivo. In wild-type cells, GAG is translated slower (7.7 codons/second) and GAA faster (18 codons/second) than the average codon (13 codons/second). Surprisingly, tRNA(Glu) lacking the 5-methylaminomethyl group, thus containing s(2)U34, translated GAA twofold faster (47 codons/second) and GAG four-fold slower (1.9 codons/second) than fully modified tRNA(Glu). In contrast, tRNA(Glu) that contains mnm(5)U34 instead of mnm(5)s(2)U34 translated GAA fourfold slower (4.5 codons/second) and GAG only 20% slower (6.2 codons/second). Clearly, the 5-methylaminomethyl group of mnm(5)s(2)U34 facilitates base-pairing with G while decreasing base-pairing with A, resulting in rates of translation of GAG and GAA that approach that of the average codon. The 2-thio group increases the recognition of GAA and has only a minor effect on the decoding of GAG. Furthermore, the 2-thio group is important for aminoacylation (see the accompanying paper). These data imply that the function of mnm(5)s(2)U34 may be different from what has been suggested previously. (C) 1998 Academic Press.