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). {ECO:0000269|PubMed:23063121, ECO:0000269|PubMed:28119416, ECO:0000269|PubMed:31329584, ECO:0000269|PubMed:31919512}. This is the function of BCDIN3D (BCDIN3 domain containing RNA methyltransferase, Ensembl gene identifier ENSG00000186666).