Probable S-adenosyl-L-methionine-dependent methyltransferase which methylates RNA molecules such as tRNAs. (Microbial infection) In case of infection by HIV-1, it binds to the loop region of TAR RNA, a region also bound by RNA polymerase II (PubMed:7638159, PubMed:8626763, PubMed:8846792). Binding of TARBP1 and RNA polymerase II to HIV-1 TAR RNA is mutually exclusive, suggesting that TARBP1 may function alone or in conjunction with HIV-1 Tat to disengage RNA polymerase II from HIV-1 TAR RNA (PubMed:7638159, PubMed:8626763, PubMed:8846792). Monomer and homodimer. Belongs to the class IV-like SAM-binding methyltransferase superfamily. RNA methyltransferase TrmH family.