Mo. Rev. Stat. § 56.360: Employment in criminal cases prohibited — civil practice authorized.

It shall be unlawful for any prosecuting attorney or circuit attorney, or any assistant prosecuting attorney or any assistant circuit attorney, during the term of office for which he shall have been elected or appointed, to accept employment by any party other than the state of Missouri in any criminal case or proceeding; provided, that nothing in this section shall be deemed to preclude the officers specified in this section from engaging in the civil practice of law. Any violation of the provisions of this section shall be deemed a misdemeanor.