Physiological Significance of the Two Isoforms of Initiator Trnas in Escherichia Coli.

Escherichia coli possesses four initiator tRNA (i-tRNA) genes, three of which are present together as metZWV and the fourth one as metY. In E. coli B, all four genes (metZWV and metY) encode i-tRNA(fMet1), in which the G at position 46 is modified to m(7)G46 by TrmB (m(7)G methyltransferase). However, in E. coli K, because of a single-nucleotide polymorphism, metY encodes a variant, i-tRNA(fMet2), having an A in place of m(7)G46. We generated E. coli strains to explore the importance of this polymorphism in i-tRNAs. The strains were sustained either on metY(A46) (metY of E. coli K origin encoding i-tRNA(fMet2)) or its derivative metY(G46) (encoding i-tRNA(fMet1)) in single (chromosomal) or plasmid-borne copies. We show that the strains sustained on i-tRNA(fMet1) have a growth fitness advantage over those sustained on i-tRNA(fMet2). The growth fitness advantages are more pronounced for the strains sustained on i-tRNA(fMet1) in nutrient-rich media than in nutrient-poor media. The growth fitness of the strains correlates well with the relative stabilities of the i-tRNAs in vivo. Furthermore, the atomistic molecular dynamics simulations support the higher stability of i-tRNA(fMet1) than that of i-tRNA(fMet2). The stability of i-tRNA(fMet1) remains unaffected upon the deletion of TrmB. These studies highlight how metY(G46) and metY(A46) alleles might influence the growth fitness of E. coli under certain nutrient-limiting conditions. IMPORTANCE Escherichia coli harbors four initiator tRNA (i-tRNA) genes: three of these at metZWV and the fourth one at metY loci. In E. coli B, all four genes encode i-tRNA(fMet1). In E. coli K, because of a single-nucleotide polymorphism, metY encodes a variant, i-tRNA(fMet2), having an A in place of G at position 46 of i-tRNA sequence in metY. We show that G46 confers stability to i-tRNA(fMet1). The strains sustained on i-tRNA(fMet1) have a growth fitness advantage over those sustained on i-tRNA(fMet2). Strains harboring metY(G46) (B mimic) or metY(A46) (K mimic) show that while in the nutrient-rich media, the K mimic is outcompeted rapidly; in the nutrient-poor medium, the K mimic is outcompeted less rapidly.