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Check out which companies are making headlines before the bell:

Post Properties — The apartment real estate investment trust is being bought by rival MAA in a $4 billion stock swap deal. The two companies focus on apartments located in Sunbelt states.

Xylem — The water treatment company is buying privately held Sensus for about $1.7 billion in cash. Sensus is a maker of smart meters and also provides data analytics services for various utility industries.

Dollar General — The retailer's stock was downgraded to "hold" from "buy" at Jefferies, both on a valuation basis and from the potential threat of highly competitive pricing.

Best Buy — Jefferies also downgraded Best Buy to "hold" from "buy," with the call based on a maturing cycle for 4K television sets.

Deckers Outdoor — The maker of UGGS and other footwear was upgraded to "outperform" from "market perform" at Telsey Advisory Group, based on the idea that weather trends should be more favorable for Deckers than in the past.

American Eagle — The apparel retailer's stock was upgraded to "buy" from "hold" at Deutsche Bank, based on valuation and the firm's expectation that American Eagle's second-quarter results will be Wall Street consensus.

Valeant Pharmaceuticals — Mizuho upgraded the drugmaker's stock to "neutral" from "underperform," based in part on more optimistic management guidance.

General Motors — Ride service company Lyft rejected buyout interest from the automaker. General Motors declined comment on the report when contacted by CNBC.

Allergan — Hedge fund manager David Tepper increased his stake in the drugmaker by 981,700 shares to a total of 1.2 million shares during the second quarter. Tepper's quarterly Securities and Exchange Commission (SEC) filing also shows a new bet on Quorum Health and a 61 percent cut in his holdings in hospital operator HCA .

Tiffany — The luxury goods retailer lost Nelson Peltz's Trian Fund as a shareholder during the second quarter, according to Trian's quarterly SEC filing. Trian took a 5.5 percent stake in Tiffany in 2007, but cut its investment in recent months and has now exited it altogether.

Facebook , Activision Blizzard — The two stocks are among new investments by hedge fund manager Daniel Loeb's Third Point, according to a quarterly SEC filing. Third Point also exited its stake in Signet Jewelers .

Tesla — Tesla removed the word "autopilot" from its China website following a Beijing crash involving a car operating in autopilot mode.

Boeing — Boeing's KC-46 refueling tanker has been cleared for production by the Defense Department. That follows a delay in the program and some redesign work.

IBM — IBM will announce a seven-year deal to provide cloud services to business software provider Workday , according to The Wall Street Journal.

Twitter — Twitter has been in discussions with Apple about bringing the Twitter app to Apple TV, according to The New York Times. Such an agreement would allow Apple TV users to view National Football League games streamed by Twitter.