Mo. Rev. Stat. § 56.550: Circuit attorneys and assistants — oaths — duties.

Before entering upon the duties of their office, the circuit attorney and said assistants shall be severally sworn to support the Constitution of the United States and the Constitution of Missouri, and to faithfully demean themselves in office. The duties of said assistants shall be to assist the circuit attorney generally in the conduct of his office, under his direction and subject to his control; and said circuit attorney and his assistants shall institute and prosecute all criminal actions in the circuit court. The circuit attorney and said assistant circuit attorneys, when so directed by the circuit attorney, may attend upon the grand jury.