Section 16. Each district attorney shall, subject to appropriation and subject to the conditions of this section, appoint and may, at his pleasure, remove such assistant district attorneys as are necessary to the functioning of the office of the district attorney. Assistant district attorneys shall receive from the commonwealth salaries as recommended by the district attorney appointing them, subject to appropriation and subject to the conditions of this section. No assistant district attorney shall be appointed and no such salary shall be paid unless and until such position and such salary (a) shall have been recommenced in writing by the district attorney making the appointment and (b) shall have been included in a schedule of offices and positions approved by the house and senate committees on ways and means. The provisions of sections nine A and forty-five of chapter thirty, chapter thirty-one, and chapter one hundred and fifty E shall not apply to said assistant district attorneys. Assistant district attorneys shall devote their full time during ordinary business hours to their duties, and shall neither directly nor indirectly engage in the practice of law.