Methyltransferase that mediates the formation of N(7)-methylguanine in a subset of RNA species, such as tRNAs, mRNAs and microRNAs (miRNAs). Catalyzes the formation of N(7)-methylguanine at position 46 (m7G46) in tRNA. Also acts as a methyltransferase for a subset of internal N(7)-methylguanine in mRNAs. Internal N(7)-methylguanine methylation of mRNAs regulates translation. Also methylates a specific subset of miRNAs. guanosine(46) in tRNA + S-adenosyl-L-methionine = N(7)-methylguanosine(46) in tRNA + S-adenosyl-L-homocysteine a guanosine in mRNA + S-adenosyl-L-methionine = an N(7)-methylguanosine in mRNA + S-adenosyl-L-homocysteine a guanosine in miRNA + S-adenosyl-L-methionine = an N(7)-methylguanosine in miRNA + S-adenosyl-L-homocysteine tRNA modification; N(7)-methylguanine-tRNA biosynthesis. Forms a complex with wdr4. Belongs to the class I-like SAM-binding methyltransferase superfamily. TrmB family.