Catalyzes the synthesis of Arg-Xaa dipeptides in an ATP-dependent manner. Has strict specificity toward arginine as the N-terminal substrate. an L-alpha-amino acid + ATP + L-arginine = ADP + H(+) + L-arginyl-L-alpha-amino acid + phosphate Activity is maximal in the presence of Mg(2+). The use of Mn(2+) or Co(2+) decreases ligase activity. Optimum pH is 9.5. Optimum temperature is 37 degrees Celsius. Homodimer.