Specifically methylates guanosine-37 in various tRNAs. guanosine(37) in tRNA + S-adenosyl-L-methionine = H(+) + N(1)-methylguanosine(37) in tRNA + S-adenosyl-L-homocysteine Homodimer. Was identified as a high-confidence drug target. Belongs to the RNA methyltransferase TrmD family.