Seven & I Holdings Co., the world’s largest convenience store franchiser, agreed to buy Marathon Petroleum Corp.’s Speedway gas stations for $21 billion, forging ahead with one of the year’s biggest deals even as the pandemic depresses economic activity in the U.S.

The transaction will add 3,900 stores to 9,800 locations operated by the retailer’s U.S.-based 7-Eleven Inc. unit, the Tokyo-based company said in a statement Monday. Seven & I shares fell as much as 8.4 percent in early trading, the biggest intraday decline since March.

The deal is the second-largest purchase of a U.S. target this year and the biggest yet for Tokyo-based Seven & I, a retail giant with 69,000 stores worldwide including 7-Eleven outlets and Ito-Yokado supermarkets in Japan.