Mo. Rev. Stat. § 66.300

The county council or other legislative authority of any first class county having a population of over six hundred thousand inhabitants is hereby authorized to impose a license tax whereby every public utility engaged in the business of supplying or furnishing electricity, electrical power, electrical service, gas, gas service, water, water service, sewer service, telegraph service or exchange telephone service in the part of the county outside incorporated cities shall pay to the county, as a license or occupational tax, an amount not in excess of five percent of the gross receipts derived from such business within the unincorporated areas of the county.