Mo. Rev. Stat. § 56.067: Prosecuting attorney must devote full time to office or special prosecutions (certain counties).

In counties of the first classification not having a charter form of government, and counties which have passed the proposition authorized by section 56.363, the prosecuting attorney, except in the performance of special prosecutions or otherwise representing the state or its political subdivisions, shall devote full time to his office, and shall not engage in the practice of law.