The district attorney shall designate one of the assistants in his district to be his first assistant district attorney and file such designation with the Secretary of State. Should the district attorney be absent, unable to attend to his duties, or disqualified to act, the first assistant district attorney may perform the duties required by law to be performed by the district attorney. In the event that the district attorney and all of his assistants are for any reason disqualified to act in a particular matter, the Attorney General of Oklahoma shall appoint a district attorney or assistant district attorney from another district to act in such matter. Whenever the office of district attorney shall be vacated by reason of resignation or death of the district attorney, the Governor of the State of Oklahoma shall appoint a qualified person to serve the balance of the unexpired term of said office.