Getco Offers to Buy Knight for $3.50 a Share The high-frequency trading firm Getco offered to buy the Knight Capital Group on Wednesday in a deal valuing the company at about $635 million, three months after Getco helped rescue Knight from the brink of bankruptcy. The buyout bid is one of at least two expected for Knight this week, with a second offer expected from another trading firm, Virtu Financial. Both companies are keenly interested in Knight's market-making trading operations, though they may sell less-desirable parts of the company. Under terms outlined in a letter to Knight's board on Wednesday, Getco proposed using cash and stock in a deal that valued Knight shares at about $3.50 apiece. Getco already owns about 24 percent of Knight. Getco's chief executive, Daniel Coleman, would hold that title in the combined company. Knight's own chairman and chief executive, Thomas M. Joyce, would become nonexecutive chairman. "I am convinced that this merger would unlock tremendous value for the shareholders of both firms while establishing a global leader in market-making and agency execution," Mr. Coleman wrote in the letter. Getco said it had lined up $950 million in financing from "a large financial institution." Shares in Knight Capital jumped more than 19 percent in premarket trading on Wednesday, to $3.54. That suggests investors expect a higher takeover bid from Getco or another bidder.