Enzymatic methylation of recombinant heterogeneous nuclear RNP protein A1. Dual substrate specificity for S-adenosylmethionine:histone-arginine N-methyltransferase. We have previously reported the existence of two different molecular species of protein methylase I (S-adenosylmethionine:protein-arginine N-methyltransferase, E.C. 2.1.1.23) in calf brain, one specific for myelin basic protein and