Folate-/FAD-dependent tRNA methyltransferase from Thermus thermophilus regulates other modifications in tRNA at low temperatures.

TrmFO is a N-5, N-10-methylenetetrahydrofolate (CH2THF)-/FAD-dependent tRNA methyltransferase, which synthesizes 5-methyluridine at position 54 (m(5)U54) in tRNA. Thermus thermophilus is an extreme-thermophilic eubacterium, which grows in a wide range of temperatures (50-83 degrees C). In T.thermophilus, modified nucleosides in tRNA and modification enzymes form a network, in which one modification regulates the degrees of other modifications and controls the flexibility of tRNA. To clarify the role of m(5)U54 and TrmFO in the network, we constructed the trmFO gene disruptant (trmFO) strain of T.thermophilus. Although this strain did not show any growth retardation at 70 degrees C, it showed a slow-growth phenotype at 50 degrees C. Nucleoside analysis showed increase in 2-O-methylguanosine at position 18 and decrease in N-1-methyladenosine at position 58 in the tRNA mixture from the trmFO strain at 50 degrees C. These invivo results were reproduced by invitro experiments with purified enzymes. Thus, we concluded that the m(5)U54 modification have effects on the other modifications in tRNA through the network at 50 degrees C. S-35 incorporations into proteins showed that the protein synthesis activity of trmFO strain was inferior to the wild-type strain at 50 degrees C, suggesting that the growth delay at 50 degrees C was caused by the inferior protein synthesis activity.