The gene for a tRNA modifying enzyme, m5U54-methyltransferase, is essential for viability in Escherichia coli

One of the most abundant modified nucleosides in tRNA is 5-methyluridine (m5U or rT, ribothymidine). The enzyme tRNA(m5U54)methyltransferase [S-adenosyl-L-methionine:tRNA (uracil-5-)-methyltransferase, EC 2.1.1.35] (the trmA gene product) catalyzes S-adenosylmethionine-dependent methylation of the uracil in position 54 (T psi C loop) in all Escherichia coli tRNAs to form m5U. Hitherto no modified nucleoside in tRNA has been shown to be essential for growth, although their importance in fine tuning the function of tRNA is well established. In this paper, we show that the structural gene trmA is essential for viability, although the known catalytic activity of the tRNA(m5U54)methyltransferase is not.