Shares of MGM Mirage ( MGG) will begin trading under symbol MGM next Monday. The Las Vegas casino operator announced the change Tuesday. Separately, MGM Mirage announced Monday evening it closed its $7.9 billion acquisition of former rival Mandalay Resort Group ( MBG). The timing of the deal's closing was expected. Just before the merger was completed, Mandalay sold its 53.5% interest in the MotorCity Casino in Detroit for about $525 million and placed into escrow its 50% interest in the Grand Victoria riverboat casino in Elgin, Ill. Both moves were required by state regulators. The merger creates what will be the world's second-largest casino company after Harrah's Entertainment ( HET) completes its pending acquisition of Caesars Entertainment ( CZR). MGM Mirage now owns and operates 24 hotel and casino properties in Nevada, Mississippi and Michigan and has investments in three other properties in Nevada, New Jersey and the U.K. The company has more than 70,000 employees. Shares were up $2.21, or 3.2%, at $71.61. Casino stocks rallied Tuesday, lifting the Dow Jones U.S. Gambling Index 2.1%, after Boyd Gaming ( BYD) reported blowout first-quarter results Monday evening. Last week, MGM Mirage reported better-than-expected first-quarter results, and Harrah's Entertainment ( HET) met Wall Street expectations after guiding sharply higher earlier in the month.