O-methyltransferase that specifically monomethylates 5'-monophosphate of cytoplasmic histidyl tRNA (tRNA(His)), acting as a capping enzyme by protecting tRNA(His) from cleavage by DICER1 (PubMed:28119416, PubMed:31329584, PubMed:31919512). Also able, with less efficiently, to methylate the 5' monophosphate of a subset of pre-miRNAs, acting as a negative regulator of miRNA processing (PubMed:23063121, PubMed:28119416). The 5' monophosphate of pre-miRNAs is recognized by DICER1 and is required for pre-miRNAs processing: methylation at this position reduces the processing of pre-miRNAs by DICER1 (PubMed:23063121). Was also reported to mediate dimethylation of pre-miR-145; however dimethylation cannot be reproduced by another group which observes a monomethylation of pre-miR-145 (PubMed:23063121, PubMed:28119416). a 5'-end (5'-phospho)-ribonucleoside-RNA + S-adenosyl-L-methionine = a 5'-end (5'-methylphospho)-ribonucleoside-RNA + S-adenosyl-L-homocysteine a 5'-end (5'-phospho)-ribonucleoside-RNA + 2 S-adenosyl-L-methionine = a 5'-end (5'-bismethylphospho)-ribonucleoside-RNA + 2 S-adenosyl-L-homocysteine Interacts with DICER1; the interaction may be mediated by RNA. Belongs to the methyltransferase superfamily. There is some controversy about O-methyltransferase on pre-miR-145, since the dimethylation first described as the specific enzymatic activity cannot be reproduced by a more recent work which observes a monomethylation of pre-miR-145 but two orders weaker than the methylation of cytosolic histidyl tRNA.