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J Bacteriol, April 1998, p. 1808-1813, Vol. 180, No. 7
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

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

Qiang Qian,1 James F. Curran,2 and Glenn R. Björk1,*

Department of Microbiology, Umeå University, S-90187 Umeå, Sweden,1 and Department of Biology, Wake Forest University, Winston-Salem, North Carolina 271092

Received 3 September 1997/Accepted 26 January 1998

tRNA species that read codons starting with adenosine (A) contain N6-threonylcarbamoyladenosine (t6A) derivatives adjacent to and 3' of the anticodons from all organisms. In Escherichia coli there are 12 such tRNA species of which two (tRNAGGUThr1 and tRNAGGUThr3) have the t6A derivative N6-methyl-N6-threonylcarbamoyladenosine (m6t6A37). We have isolated a mutant of E. coli that lacks the m6t6A37 in these two tRNAGGUThr species. These tRNA species in the mutant are likely to have t6A37 instead of m6t6A37. We show that the methyl group of m6t6A37 originates from S-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-tRNAGGUThr 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 m6t6A37-containing tRNAGGUThr species. 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 tRNAGGUThr slightly reduces the efficiency of this tRNA to read cognate codon ACC.

* Corresponding author. Mailing address: Department of Microbiology, Umeå University, S-901 87 Umeå, Sweden. Phone: 46-90-7856756. Fax: 46-90-772630. E-mail: Glenn.Bjork{at}micro.umu.se.




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