Abstract
More than 90 different modified nucleosides have been identified in tRNA. Among the tRNA modifications, the 7-methylguanosine (m7G) modification is found widely in eubacteria, eukaryotes, and a few archaea. In most cases, the m7G modification occurs at position 46 in the variable region and is a product of tRNA (m7G46) methyltransferase. The m7G46 modification forms a tertiary base pair with C13-G22, and stabilizes the tRNA structure. A reaction mechanism for eubacterial tRNA m7G methyltransferase has been proposed based on the results of biochemical, bioinformatic, and structural studies. However, an experimentally determined mechanism of methyl-transfer remains to be ascertained. The physiological functions of m7G46 in tRNA have started to be determined over the past decade. For example, tRNA m7G46 or tRNA (m7G46) methyltransferase controls the amount of other tRNA modifications in thermophilic bacteria, contributes to the pathogenic infectivity, and is also associated with several diseases. In this review, information of tRNA m7G modifications and tRNA m7G methyltransferases is summarized and the differences in reaction mechanism between tRNA m7G methyltransferase and rRNA or mRNA m7G methylation enzyme are discussed.
Highlights
Transfer RNA, which is one of the classical non-coding RNAs, functions as an adaptor molecule supplying amino acids to ribosomes according to the codon of the mRNA
It is thought that m7G is generated immediately after the transcription. m7G is most frequently located at position 46 in the tRNA variable region, and forms a tertiary base pair with C13-G22 in the three-dimensional core of tRNA
With respect to the relationship with rRNA m7G methyltransferase, neither Trm8 nor Trm82 has significant similarity to KgmB (the kanamycin-gentamicin resistance methylase, 16S rRNA (m7G1405) methyltransferase) from Streptomyces tenebrarius, a rRNA m7G methyltransferase associated with aminoglycoside resistance [36,37], other than the methyltransferase domain shared by numerous methyltransferases [75]
Summary
Transfer RNA, which is one of the classical non-coding RNAs, functions as an adaptor molecule supplying amino acids to ribosomes according to the codon of the mRNA. The modified nucleosides include thiolation, deamination, isomerization conversion of uridine to pseudouridine, or the combination of several modifications. 7-methylguanosine (m7G) is one of the most conserved modified nucleosides and is common in eubacteria, eukaryotes [4], and a few archaea [7]. M7G is present in introns containing pre-tRNA together with N2,N2-dimethylguanine (m22G), Ψ and m1A [9]. For this reason, it is thought that m7G is generated immediately after the transcription. M7G is most frequently located at position 46 in the tRNA variable region, and forms a tertiary base pair with C13-G22 in the three-dimensional core of tRNA
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