SOUTH PORTLAND — The Maine company Wright Express Corp. is expanding its presence in the heavy truck market by acquiring fuel card company Fleet One from private equity firms for $369 million.

The South Portland-based company announced Wednesday it is has entered into an agreement to buy Fleet One from LLR Partners and FTV Capital in an all-cash deal that’s expected to close later this year.

Wright Express processes fuel payments for fleet vehicles. Its charge cards are used by commercial and government fleets to purchase fuel and maintenance services for about 4.5 million vehicles. The company had more than $550 million in revenues in 2011.

Nashville-based Fleet One also provides fuel cards and fleet management information services. It had revenues of more than $56 million in the 12 months ending June 30.