N-7-Methylguanine at position 46 (m(7)G46) in tRNA from Thermus thermophilus is required for cell viability at high temperatures through a tRNA modification network

N-7-methylguanine at position 46 (m(7)G46) in tRNA is produced by tRNA (m(7)G46) methyltransferase (TrmB). To clarify the role of this modification, we made a trmB gene disruptant (delta trmB) of Thermus thermophilus, an extreme thermophilic eubacterium. The absence of TrmB activity in cell extract from the delta trmB strain and the lack of the m(7)G46 modification in tRNA(Phe) were confirmed by enzyme assay, nucleoside analysis and RNA sequencing. When the delta trmB strain was cultured at high temperatures, several modified nucleotides in tRNA were hypo-modified in addition to the lack of the m(7)G46 modification. Assays with tRNA modification enzymes revealed hypo-modifications of Gm18 and m(1)G37, suggesting that the m(7)G46 positively affects their formations. Although the lack of the m(7)G46 modification and the hypo-modifications do not affect the Phe charging activity of tRNA(Phe), they cause a decrease in melting temperature of class I tRNA and degradation of tRNA(Phe) and tRNA(Ile). S-35-Met incorporation into proteins revealed that protein synthesis in delta trmB cells is depressed above 70 degrees C. At 80 degrees C, the delta trmB strain exhibits a severe growth defect. Thus, the m(7)G46 modification is required for cell viability at high temperatures via a tRNA modification network, in which the m(7)G46 modification supports introduction of other modifications.