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    Tetraphase shares unchanged on first day of trading after $75 million IPO

    Tetraphase Pharmaceuticals Inc. stock closed unchanged at $7 a share on its first day of trading Wednesday after the Watertown biotechnology start-up raised $74.9 million in an IPO.

    Tetraphase, which is working on technology to create antibiotics for abdominal and urinary tract infections, priced its offering of 10.7 million shares of common stock at $7 a share. That was below the $10 to $12 price range initially specified in the company’s registration statement.

    Another Watertown biotechnology company, Enanta Pharmaceuticals Inc., priced 4 million shares at $14 per share Wednesday in what would be a $56 million IPO. Enanta, which is developing drugs to treat the hepatitis C virus, will begin trading Thursday.

    Robert Weisman

    Robert Weisman can be reached at Follow him on Twitter @GlobeRobW.