Ameren ( AEE), a St. Louis-based public utility company, said its unit AmerenUE has completed the purchase of the 640-megawatt Audrain combustion turbine generating station, a gas-fired peaking facility in Vandalia, Mo. The station was bought from affiliates of NRG Energy ( NRG) for $115 million. The Federal Energy Regulatory Commission and the Missouri Public Service Commission granted approval for the deal, and the antitrust waiting period has expired. Ameren said the facility will help the company meet increased needs for generating capacity and provide additional flexibility in determining future capacity requirements. Shares of Ameren were trading up 5 cents at $50.50 Wednesday.