The Methyl Group of theN6-Methyl-N6-Threonylcarbamoyladenosine in tRNA of Escherichia coli Modestly Improves the Efficiency of the tRNA

ABSTRACT tRNA species that read codons starting with adenosine (A) containN 6-threonylcarbamoyladenosine (t6A) derivatives adjacent to and 3′ of the anticodons from all organisms. InEscherichia coli there are 12 such tRNA species of which two (tRNAGGU Thr1 and tRNAGGU Thr3) have the t6A derivativeN 6-methyl-N 6-threonylcarbamoyladenosine (m6t6A37). We have isolated a mutant ofE. coli that lacks the m6t6A37 in these two tRNAGGU Thr species. These tRNA species in the mutant are likely to have t6A37 instead of m6t6A37. We show that the methyl group of m6t6A37 originates fromS-adenosyl-l-methionine and that the gene (tsaA) which most likely encodes tRNA(m6t6A37)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-tRNAGGU Thr to the ribosomal A site programmed with either of the cognate codons ACC and ACU were the same for thetsaA1 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 m6t6A37-containing tRNAGGU Thrspecies. We show that the tsaA1 mutation resulted in a twofold derepression of this operon, suggesting that the lack of the methyl group of m6t6A37 in tRNAGGU Thr slightly reduces the efficiency of this tRNA to read cognate codon ACC.