Section 5. There shall be in the department a deputy commissioner who shall also be the secretary. He shall be appointed by the commissioner, with the approval of the governor and council, and may, with like approval, be removed at the pleasure of the commissioner. The deputy commissioner, in addition to his duties as secretary under section twelve, shall perform such other duties as may be assigned to him by the commissioner, and during the illness, absence or other disability of the commissioner, he shall perform the official duties of the commissioner.