Specifically methylates the guanosine in position 745 of 23S rRNA. guanosine(745) in 23S rRNA + S-adenosyl-L-methionine = H(+) + N(1)-methylguanosine(745) in 23S rRNA + S-adenosyl-L-homocysteine Homodimer. Belongs to the methyltransferase superfamily. RlmA family.