1-Methylguanosine deficiency of tRNA influences cognate codon interaction and metabolism in Salmonella typhimurium

Abstract

1-Methylguanosine (m1G) is present next to the 3' end of the anticodon (position 37) in tRNA(1,2,3,Leu), tRNA(1,2,3,Pro), and tRNA(3Arg). A mutant of Salmonella typhimurium lacks m1G in these seven tRNAs when grown at or above 37 degrees C, as a result of a mutation (trmD3) in the structural gene (trmD) for the tRNA(m1G37)methyltransferase. The m1G deficiency induced 24 and 26% reductions in the growth rate and polypeptide chain elongation rate, respectively, in morpholinepropanesulfonic acid (MOPS)-glucose minimal medium at 37 degrees C. The expression of the leuABCD operon is controlled by the rate with which tRNA(2Leu) and tRNA(3Leu) read four leucine codons in the leu-leader mRNA. Lack of m1G in these tRNAs did not influence the expression of this operon, suggesting that m1G did not influence the efficiency of tRNA(2,3Leu). Since the average step time of the m1G-deficient tRNAs was increased 3.3-fold, the results suggest that the impact of m1G in decoding cognate codons may be tRNA dependent. The trmD3 mutation rendered the cell more resistant or sensitive to several amino acid analogs. 3-Nitro-L-tyrosine (NT), to which the trmD3 mutant is sensitive, was shown to be transported by the tryptophan-specific permease, and mutations in this gene (mtr) render the cell resistant to NT. Since the trmD3 mutation did not affect the activity of the permease, some internal metabolic step(s), but not the uptake of the analog per se, is affected. We suggest that the trmD3-mediated NT sensitivity is by an abnormal translation of some mRNA(s) whose product(s) is involved in the metabolic reactions affected by the analog. Our results also suggest that tRNA modification may be a regulatory device for gene expression.