S-adenosyl-L-methionine-dependent 2'-O-ribose methyltransferase that catalyzes the formation of 2'-O-methyluridine at position 2791 (Um2791) in the 21S mitochondrial large subunit ribosomal RNA (mtLSU rRNA), a universally conserved modification in the peptidyl transferase domain of the mtLSU rRNA. S-adenosyl-L-methionine + uridine(2791) in 21S rRNA = 2'-O-methyluridine(2791) in 21S rRNA + H(+) + S-adenosyl-L-homocysteine Loses mitochondrial DNA with high frequency. Belongs to the class I-like SAM-binding methyltransferase superfamily. RNA methyltransferase RlmE family.