Crystal structure of tRNA (Um34/Cm34) methyltransferase TrmL from Escherichia coli
Classification:
TRANSFERASE
Technique:
X-Ray Diffraction
Resolution:
2.0
R value free:
0.204
R value observed:
0.19
R value work:
0.189
Abstract of the PDB Structure's related Publication:
Unlike other transfer RNAs (tRNA)-modifying enzymes from the SPOUT methyltransferase superfamily, the tRNA (Um34/Cm34) methyltransferase TrmL lacks the usual extension domain for tRNA binding and consists only of a SPOUT domain. Both the catalytic and tRNA recognition mechanisms of this enzyme remain elusive. By using tRNAs purified from an Escherichia coli strain with the TrmL gene deleted, we found that TrmL can independently catalyze the methyl transfer from S-adenosyl-L-methionine to and isoacceptors without the involvement of other tRNA-binding proteins. We have solved the crystal structures of TrmL in apo form and in complex with S-adenosyl-homocysteine and identified the cofactor binding site and a possible active site. Methyltransferase activity and tRNA-binding affinity of TrmL mutants were measured to identify residues important for tRNA binding of TrmL. Our results suggest that TrmL functions as a homodimer by using the conserved C-terminal half of the SPOUT domain for catalysis, whereas residues from the less-conserved N-terminal half of the other subunit participate in tRNA recognition.
This small SAM-dependent methyltransferase (157 amino acids) exists as an homodimer. It acts on the ribose of the wobble U or C34 of E. coli tRNALeu harboring an i6A37 modification in the anticodon loop. It allows increasing the performance of the tRNA reading the corresponding complementary codons. The enzyme is not essential for cell viability. Belongs to the SPOUT-type TrmH/TrmL subfamily.