Mo. Rev. Stat. § 386.110
Each commissioner and each person appointed to office or employment by the governor or by the commission shall, before entering upon the duties of the commissioner's office or employment, take and subscribe to an oath or affirmation to support the Constitution of the United States and of this state, and to faithfully and honestly discharge the duties of such office. No person shall be eligible to appointment or shall hold the office of commissioner, or be appointed by the commission, or hold any office or position under the commission, who holds any official relation to any gas corporation, electrical corporation, telephone corporation, telegraph corporation, water corporation, heat and refrigerating corporation, sewer corporation, or other public service or public utility corporation or person subject to any of the provisions of this chapter, or who owns stocks or bonds therein, or who has any pecuniary interest therein.
§ 386.110, RSMo