7-Methylguanosine Modifications in Transfer RNA (tRNA)

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.

Keywords: RNA modification; methylase; tRNA methyltransferase; tRNA modification.

Figure 1

tRNA m⁷G46 methyltransferase methylates the N7-atom of guanine at position 46 in tRNA and forms m⁷G46. (A) The secondary structure of tRNA is presented in cloverleaf form. Conserved nucleotides are depicted as follows: adenosine, A; guanosine, G; cytidine, C; uridine, U; purine, R; pyrimidine, Y. tRNA (m⁷G46) methyltransferase transfers a methyl-group to the N7-atom of guanine at position 46 in tRNA and forms 7-methylguanine. (B) The L-shaped structure of tRNA is presented. The m⁷G46 forms a tertiary base pair with the C13-G22 base pair in the L-shaped tRNA structure. In the stick model, only atoms are visible and charge is invisible.

Figure 2

Positions of the m⁷G modification in tRNA. The numbers in circles indicate the positions of m⁷G in tRNA.

Figure 3

Comparison of thermophilic and methophilic TrmB. (A) Sequence alignment of TrmB. Conserved regions are highlighted in three colored squares (red, green, blue). Asp133 is indicated by an asterisk. (B) Thermophilic and mesophilic TrmB proteins are illustrated schematically. The three colored regions correspond to the amino acid sequences in panel A. Thermophilic TrmB has a distinct long C-terminal region. Asp133 is highlighted with an arrow, and the red D corresponds to the Asp133.

Figure 4

The amino acid residues for AdoMet binding and hypothetical reaction mechanism of TrmB. (A) The amino acid residues (E47, Y95, R108, T165 and Y167) involved in AdoMet binding (orange) are indicated on the catalytic domain of B. subtilis TrmB structure (PDB: 2FCA). The hypothetical catalytic center (grey shading), Asp133 (red) and the other important amino acid residues (D74, D97 and T132; blue) are highlighted. (B) Hypothetical reaction mechanism of eubacterial TrmB proteins is drawn.