Mo. Rev. Stat. § 28.190: Vacancy — governor's duties.

In case of death, resignation, removal from office, impeachment or vacancy from any cause in the office of secretary of state, the governor shall immediately appoint a qualified person to fill such vacancy for the remainder of the term in which such vacancy occurred and until his successor is elected or appointed, commissioned and qualified; and the governor shall take charge of the office and superintend its business until such person is appointed, commissioned and qualified; except that in case of impeachment, the governor shall appoint a qualified person to serve only until such impeachment is determined, when the suspended officer, if acquitted, shall be reinstated in office, or if the suspended officer is convicted, a new appointment shall be made by the governor as in the case of other vacancies.