7-Methylguanosine Modifications in tRNA
Summary (3 min read)
- 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.
- 7-methylguanosine (m7G) is one of the most conserved modified nucleosides and is common in eubacteria, eukaryotes , and a few archaea .
- The m7G46 forms a tertiary base pair with the C13-G22 base pair in the L-shaped tRNA structure.
- Domains of life Organisms Positions of m7G Enzyme names Higher order structure References eubacteria E. coli 46 TrmB monomer [51,56] A. aeolicus 46 TrmB monomer [49,57,58].
- By combining the aniline cleavage method and the Donis-Keller-method, which uses ribonucleases [42,43], it is possible to identify the position of m7G in tRNA.
2. Structural Analyses and Catalytic Mechanisms of m7G Methyltransferases
- Structural studies of tRNA modification enzymes can be informative both for the specificity and catalytic mechanism of the enzymes.
- Furthermore, the combination of structural and biochemical analysis data allows comparison of reaction mechanisms from different species.
- This makes it possible to infer information about molecular evolution.
- MTases belong to two protein super families, which are structurally and phylogenetically unrelated, namely, the Rossmann fold MTases (RFM) and SPOUT MTases (SpoU and TrmD) .
2.1. Eubacterial tRNA m7G46 Methyltransferases (TrmB)
- Dimerization does not seem to be a common feature of the enzymes, as the interface is not conserved and the TrmB enzymes of E. coli and A. aeolicus are monomeric.
- Alanine substitution of Arg287 in the long C-terminal region considerably reduces the methyltransfer activity.
- Figure 3. Comparison of thermophilic and methophilic TrmB. (A) Sequence alignment of TrmB.
- The noncatalytic subunits are involved in stabilizing the catalytic subunit or activating and fine tuning the activity.
- Separate expression of Trm8 and Trm82 proteins and their subsequent mixing resulted in proteins with no activity.
2.2. Heterodimeric tRNA m7G Methyltransferase of Yeast (Trm8/Trm82)
- Yeast Tr 8/Trm82 proteins are unrelated and have no homology to each other.
- The homologous human proteins METTL/WDR4 complement Trm8/Trm82 in yeast .
- The targets of these methyltransferase are components of different parts of the translation machinery, namely, rRNA, tRNAs, and release factors .
- The noncatalytic subunits ar involved in stabilizing the catalytic subunit or activating and fine tuni g the activity.
- An active Trm8/Trm82 heterodimer was synthesized when RNAs of both Trm8 and Trm82 were co-translat d .
2.3. mRNA Cap m7G Methyltransferase
- TrmB and mRNA cap-m7G methyltransferase (Abd1) have different targets for methylation.
- In the catalytic center of TrmB, the amino acid residues present differ totally from those in the catalytic center of Abd1.
- The m7G methyltransferase domain is heterodimerized with a stimulatory vD12 subunit [73,74].
- An allosteric mechanism, whereby the vD12 subunit enhances the affinity of the catalytic vD1 subunit for AdoMet and the guanine acceptor, has been proposed.
- It has been shown that the catalytic subunits of vD1, as well as the yeast mRNA capping enzyme Abd1, are unrelated to Trm8.
3. Physiological Functions
- A large number of tRNA modifications have important roles in tRNA function .
- In particular, tRNA modifications in the anticodon region play a major role in translation and growth .
- The role of many tRNA modifications outside of the anticodon region are considered auxiliary to correct structure formation and fine tuning of the translation because it hardly appears as phenotypic defects [76,77].
- Because of this, information about the role of the m7G46 modification in tRNA was limited for a long time, even though the modification is widely found in eubacteria and eukaryotes.
- Clarification of the function of m7G46 in tRNA has begun over the past decade.
3.1. tRNA m7G46 Modification in Yeast
- TrmB gene disruption in E. coli demonstrated no phenotypic defects .
- Since both Trm8 and Trm82 are absolutely required to form m7G in yeast, a phenotype would be expected already in trm8 or trm82 single mutants and not requiring a double deletion .
- Hypo-modified mature tRNAVal(AAC) deacylates and degrades rapidly in a double deletion mutant strain of trm8 and trm4 (∆trm4∆trm8), (Trm4 is a methyltransferase for 5-methyl cytidine at positions 34, 40, 48, and 49 in tRNA) at 37 ◦C, resulting in a temperature sensitive phenotype .
- Deletion of MET22, which likely regulates 5′–3′ exonuclease Rat1 and Xrn1 activity indirectly, prevents tRNAVal(AAC) degradation in the ∆trm4∆trm8 strain.
- Maf1 inhibits tRNA transcription via a mechanism dependent on phosphorylation and nuclear accumulation of Maf1, followed by physical association with Pol III in the tRNA genes.
3.2. tRNA m7G46 Modification in Thermophilic Eubacteria
- In comparison with these eukaryotic enzymes, there is limited information about eubacterial enzymes.
- The mechanism by which modifications are controlled remained unknown until the beginning of the 21st century.
- When the trmB gene was disrupted, the introduction ratio of Gm18, m5s2U54, and m1A58 was dramatically changed, and the melting temperature of the hypo-modified tRNA decreased.
- In particular, degradation of tRNAPhe and tRNAIle was detected.
- Thus, the m7G46 and Ψ55 modifications work as an accelerator and a brake, respectively .
3.3. Involvement of tRNA m7G46 Modification in Fungal Pathogenicity
- The first report of the relationship between tRNA modification enzymes and fungal pathogenicity was by Takano et al., who showed that the tRNA m7G46 modification is required for plant infection by the phytopathogenic fungus Colletotrichum lagenarium, the cause of cucumber anthracnose .
- Aph1 (Appressorial Penetration into Host) is required for efficient tRNA m7G46 modification in C. lagenarium, and experiments with aph1 gene knockout mutants suggest that Aph1 is required for appressorium-mediated host invasion and also has important roles in resistance to several stresses including the basic defense response of the host plant.
- Given that in addition to m7G46 there are other tRNA modifications which are related to infection , tRNA modification and tRNA modification enzymes are likely be an important factor in the relationship between host and infectious organisms.
3.4. Involvement of tRNA m7G46 Methyltransferase in Diseases
- Since tRNA modification regulates protein synthesis, there are several reports on the relationship between tRNA modification and genetic disease.
- METTL1 and WDR4 are the human homologues of Trm8 and Trm82, respectively.
- The observation of the influence of 5-FU in the yeast ∆trm8 strain leads to the hypothesis that these RTD-rerated modifying enzymes might affect the efficiency of 5-FU in human cancer cells.
- Thus, NSUN2 and METTL1 are involved 5-FU sensitivity in HeLa cells.
- This study has clearly demonstrated the tRNA m7G methylome in mammals and shows the critical nature of METTL1 and WDR and m7G modification in regulation of stem cells and disease.
- Since m7G was found in tRNA, the genes encoding tRNA m7G methyltransferase have been identified in several organisms, and amino acid residues key to the reaction mechanism have been identified [22,25,29,57].
- Recently, a reaction mechanism for TrmB has been proposed .
- Also, genes encoding the tRNA m7G methyltransferase responsible for m7G at position 49 in archaeal tRNA and for anticodon m7G of mt tRNA or chloroplast tRNA have not yet been identified.
- This work was supported by a Grant-in-Aid for Scientific Research (16K18493 to C.T.) from the Japan Society for the Promotion of Science (JSPS).
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...occurring most commonly in position 46 of the tRNAs in both yeast and human and position 1639 of the 18S subunit of the human rRNA [70-72]....
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Q1. What contributions have the authors mentioned in the paper "7-methylguanosine modifications in transfer rna (trna)" ?
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.