Mitochondrial S-adenosyl-L-methionine-dependent methyltransferase that catalyzes the formation of 5-methyl-uridine in tRNAs and 12S rRNA. Catalyzes the methylation of uridine at position 54 (m5U54) in all tRNAs. Specifically methylates the uridine in position 429 of 12S rRNA (m5U429). Does not affect RNA stability or mitochondrial translation.
Subcellular locations: Mitochondrion matrix