78.320.  Tenure — qualifications. — In cities wherein civil service commissioners have been appointed under the provisions of sections 78.010 to 78.420, each succeeding council shall as soon as practicable after organizing appoint one commissioner for four years who shall take the place of the commissioner whose term of office expires.  The chairman of the commission for each biennial period shall be the member whose term first expires.  No person while on the said commission shall hold or be a candidate for any office of public trust.  Two members shall constitute a quorum to transact business. The commissioners must be citizens of Missouri and residents of the city for more than three years next preceding their appointment.