THE METHYL-GROUP OF THE N-6-METHYL-N-6-THREONYLCARBAMOYLADENOSINE IN TRANSFER-RNA OF ESCHERICHIA-COLI MODESTLY IMPROVES THE EFFICIENCY OF THE TRANSFER-RNA

tRNA species that read codons starting with adenosine (A) contain N-6- threonylcarbamoyladenosine (t(6)A) derivatives adjacent to and 3' of t he anticodons from all organisms, In Escherichia coli there are 12 suc h 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)A3 7 in these two tRNA(GGU)(Thr) species. These tRNA species in the mutan t 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 an d that the gene (tsaA) which most likely encodes tRNA(m(6)t(6)A37)meth yltransferase is located at min 4.6 on the E. coli chromosomal map. Th e 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 t saA1 mutant as for the congenic wild-type strain, The expression of th e 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-c ontaining tRNA(GGU)(Thr) species. We show that the tsaA1 mutation resu lted in a twofold derepression of this operon, suggesting that the lac k of the methyl group of m(6)t(6)A37 in tRNA(GGU)(Thr) slightly reduce s the efficiency of this tRNA to read cognate codon BCC.