Atlantic Capital Bancshares said it is acquiring Chattanooga-based First Security Group in a deal that will double its size and set the stage for a planned public stock offering.

With the $160 million stock-and-cash deal, announced Wednesday, the Atlanta bank will be picking up a community banking operation with $1.1 billion in assets and 26 branches in Tennessee and north Georgia.

In contrast, Atlantic Capital has only one office. But since launching in 2007 — one of the last bank start-ups in Georgia before the 2008 financial crisis turned the state into the nation’s epicenter for bank failures — Atlantic Capital has grown to $1.3 billion in assets with a focus on serving mid-sized businesses and their managers and owners.

Atlantic Capital said it plans to keep corporate offices in both Atlanta and Chattanooga after the merger.

“Our goal is to build a premier financial institution in the Southeast by focusing on business and private banking,”

said Atlantic Capital CEO Douglas Williams, who will head the merged banks. “Each bank brings complementary strengths that will only be enhanced by the combined size and geographic reach of the merger. The merger will provide the foundation for a strong financial institution that will produce solid returns for our shareholders.”

Atlantic Capital said it expects to complete the merger deal in the fall, subject to regulators’ and shareholders’ approval.

At the same time, to help finance the deal, Atlantic Capital said it expects to complete a $25 million sale of stock to Stone Point Capital, a Greenwich, Conn., private equity fund. With the closing of the merger, Atlantic Capital also said it plans to take the company public by registering its shares with the U.S. Securities and Exchange Commission and seeking a listing on the NASDAQ stock market.