Two-subunit enzymes involved in eukaryotic post-transcriptional tRNA modification

Abstract

tRNA modifications are crucial for efficient and accurate protein translation, with defects often linked to disease. There are 7 cytoplasmic tRNA modifications in the yeast Saccharomyces cerevisiae that are formed by an enzyme consisting of a catalytic subunit and an auxiliary protein, 5 of which require only a single subunit in bacteria, and 2 of which are not found in bacteria. These enzymes include the deaminase Tad2-Tad3, and the methyltransferases Trm6-Trm61, Trm8-Trm82, Trm7-Trm732, and Trm7-Trm734, Trm9-Trm112, and Trm11-Trm112. We describe the occurrence and biological role of each modification, evidence for a required partner protein in S. cerevisiae and other eukaryotes, evidence for a single subunit in bacteria, and evidence for the role of the non-catalytic binding partner. Although it is unclear why these eukaryotic enzymes require partner proteins, studies of some 2-subunit modification enzymes suggest that the partner proteins help expand substrate range or allow integration of cellular activities.

Keywords: Tad2; Trm112; Trm61; Trm7; Trm734; Trm8; Trm9; cerevisiae; modification; tRNA.

Figure 1.

S. cerevisiae tRNA modifications formed by 2-subunit enzymes are located throughout the tRNA. Cloverleaf schematic depicting tRNA residues known to be modified (black) in S. cerevisiae, and those that are not modified (gray). Modifications (in brackets) formed by 2-subunit enzymes are labeled.

Figure 2.

Schematic of FtsJ methyltransferase stem-loop substrates. S. cerevisiae Trm7 requires Trm732 for Cm₃₂ modification and Trm734 for Gm₃₄ modification of the anticodon loop of tRNA^Phe which, as for all tRNAs, has 7 bases. These modifications then drive formation of yW₃₇ from its m¹G precursor. The thicker arrow from Gm₃₄ indicates that yW formation is more dependent on this modification. Other FtsJ family members modify the 5-base A-loop in the rRNA large subunit in different organisms and organelles, as indicated.

Figure 3.

Trm112 partners with several methyltransferases involved in diverse translational processes. Schematic of the S. cerevisiae Trm112 physical interactions known to affect the activity or stability (in brackets) of its partner methyltransferases. (*) The Sc Bud23 protein, but not its methyltransferase activity, is required for m⁷G₁₅₇₅ formation on the cytoplasmic 18S rRNA subunit.