The Methyl Group of the N(6)-Methyl-N(6)-Threonylcarbamoyladenosine in tRNA of Escherichia coli Modestly Improves the Efficiency of the tRNA

tRNA species that read codons starting with adenosine (A) contain N(6)-threonylcarbamoyladenosine (t(6)A) derivatives adjacent to and 3′ of the anticodons from all organisms. In Escherichia coli there are 12 such tRNA species of which two (tRNA(GGU)(Thr1) and tRNA(GGU)(Thr3)) have the t(6)A derivative N(6)-methyl-N(6)-threonylcarbamoyladenosine (m(6)t(6)A37). We have isolated a mutant of E. coli that lacks the m(6)t(6)A37 in these two tRNA(GGU)(Thr) species. These tRNA species in the mutant are likely to have t(6)A37 instead of m(6)t(6)A37. We show that the methyl group of m(6)t(6)A37 originates from S-adenosyl-l-methionine and that the gene (tsaA) which most likely encodes tRNA(m(6)t(6)A37)methyltransferase is located at min 4.6 on the E. coli chromosomal map. The growth rate of the cell, the polypeptide chain elongation rate, and the selection of Thr-tRNA(GGU)(Thr) to the ribosomal A site programmed with either of the cognate codons ACC and ACU were the same for the tsaA1 mutant as for the congenic wild-type strain. The expression of the threonine operon is regulated by an attenuator which contains in its leader mRNA seven ACC codons that are read by these two m(6)t(6)A37-containing tRNA(GGU)(Thr) species. We show that the tsaA1 mutation resulted in a twofold derepression of this operon, suggesting that the lack of the methyl group of m(6)t(6)A37 in tRNA(GGU)(Thr) slightly reduces the efficiency of this tRNA to read cognate codon ACC.