Does the Department of Justice (DOJ) have legal authority to be representing the Internal Revenue Service (IRS)?

There is evidence from the DOJ and Treasury Department that the DOJ has NO authority from Congress to be defending the IRS or prosecuting for the IRS in any proceedings. Absent any laws authorizing said functions, they are acting under the color of law, and all court activity and trials involving the DOJ and IRS are likely void for lack of jurisdiction!