Mark Twain Bancshares said Monday that it was in merger negotiations, prompting widespread speculation that it was about to be acquired by First Bank System Inc.

No deal had been announced by the close of trading, but Mark Twain shares gained $2.875 to close at a 52-week high of $37.875 after the St. Louis bank notified the National Association of Securities Dealers that it "has been in negotiations with respect to a possible strategic merger."

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