Essential arginine methyltransferase that can both catalyze the formation of omega-N monomethylarginine (MMA) and symmetrical dimethylarginine (sDMA). Specifically mediates the symmetrical dimethylation of arginine residues in the small nuclear ribonucleoproteins SmD1 and SmD3 (By similarity). Belongs to the class I-like SAM-binding methyltransferase superfamily. Protein arginine N-methyltransferase family. PRMT7 subfamily.