S-adenosyl-L-methionine-dependent 2'-O-ribose methyltransferase that catalyzes the formation of 2'-O-methylguanosine at position 2270 (Gm2270) in the 21S mitochondrial large subunit ribosomal RNA (mtLSU rRNA), a universally conserved modification in the peptidyl transferase domain of the mtLSU rRNA. This modification seems to be important for the normal accumulation of the mitochondrial large ribosomal subunit. guanosine(2270) in 21S rRNA + S-adenosyl-L-methionine = 2'-O-methylguanosine(2270) in 21S rRNA + H(+) + S-adenosyl-L-homocysteine Present with 2760 molecules/cell in log phase SD medium. Belongs to the class IV-like SAM-binding methyltransferase superfamily. RNA methyltransferase TrmH family.