Mitochondrial S-adenosyl-L-methionine-dependent methyltransferase that catalyzes the formation of 5-methyl-uridine in tRNAs and 12S rRNA (PubMed:31948311, PubMed:34556860). Catalyzes the methylation of uridine at position 54 (m5U54) in all tRNAs (PubMed:31948311). Specifically methylates the uridine in position 429 of 12S rRNA (m5U429) (PubMed:31948311). Does not affect RNA stability or mitochondrial translation (PubMed:31948311).
