Catalyzes the formation of 5-methyl-uridine at position equivalent to 747 (m5U747) in 23S rRNA (m5U859 in the P.abyssi numbering). S-adenosyl-L-methionine + uridine(747) in 23S rRNA = 5-methyluridine(747) in 23S rRNA + H(+) + S-adenosyl-L-homocysteine Activated by magnesium ions. Belongs to the class I-like SAM-binding methyltransferase superfamily. RNA M5U methyltransferase family.