The county legislative body is authorized to amend zoning regulations, although any amendment must first be submitted to the regional planning commission, which has thirty (30) days to pass the amendment or to offer suggestions.  If the planning commission disapproves, the amendment becomes effective only through a subsequent majority vote of the county legislative body.  Before final adoption, the county legislative body must hold a public hearing, giving at least fifteen (15) days notice (thirty (30) days in Shelby County) in a newspaper of general circulation in the county and including a summary of the proposed amendment.  T.C.A. § 13-7-105.