Iowa City-based MidwestOne Financial Group Inc. will expand its banking operations into Greater Des Moines with the acquisition of Dubuque-based AT Bancorp. The bank holding companies said today they have agreed to a merger valued at $170.3 million, based on MidwestOne’s stock price of $32.92 per share on Monday. AT Bancorp is the parent of American Trust and Savings Bank, which has branches on University Avenue in West Des Moines and Windsor Heights and a commercial lending office on Bishop Drive in West Des Moines. MidwestOne will be the surviving institution after the acquisition, which will create an operation with $4.7 billion assets with 60 branches in Iowa, Minnesota, Wisconsin, Florida and Colorado, according to a release.