Methyltransferase that mediates the formation of N(7)-methylguanine in a subset of RNA species, such as tRNAs, mRNAs and microRNAs (miRNAs) (PubMed:29983320). 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 (PubMed:29983320). Internal N(7)-methylguanine methylation of mRNAs regulates translation (PubMed:29983320). Also methylates a specific subset of miRNAs, such as let-7. N(7)-methylguanine methylation of let-7 miRNA promotes let-7 miRNA processing by disrupting an inhibitory secondary structure within the primary miRNA transcript (pri-miRNA) (By similarity). Acts as a regulator of embryonic stem cell self-renewal and differentiation (PubMed:29983320). 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. Phosphorylation at Ser-21 inactivates its catalytic activity but does not affect the interaction with WDR4. Belongs to the class I-like SAM-binding methyltransferase superfamily. TrmB family.