Mo. Rev. Stat. § 56.240
The prosecuting attorney in counties of the third class may appoint assistant prosecuting attorneys who shall be licensed attorneys in the state of Missouri and be subject to all the liabilities and penalties for failure or neglect to discharge the duty to which prosecuting attorneys are liable. The appointment of an assistant prosecuting attorney shall be made in writing and filed by the prosecuting attorney, and such assistant prosecuting attorney shall take and subscribe to the oath or affirmation of office required of prosecuting attorneys, which appointment and oath or affirmation of office shall be filed in the office of the clerk of the circuit court of the county. An assistant prosecuting attorney shall discharge the duties of the prosecuting attorney when the prosecuting attorney is sick or absent from the county, or when the prosecuting attorney is engaged in the discharge of the duties of his office so that he cannot attend. In counties of the third class an assistant prosecuting attorney shall assist the prosecuting attorney in any case when requested to do so by the prosecuting attorney, but the former shall be disqualified from defending in any criminal case. The compensation of an assistant prosecuting attorney in third class counties may be paid out of the county treasury an annual salary in an amount approved by the county commission.
§ 56.240, RSMo