NEW YORK (AP) -- Sprint Nextel Corp. is intensifying its focus on the fast-growing market for prepaid cell phone service with a $483 million deal to buy Virgin Mobile USA Inc.

The acquisition announced Tuesday calls for Sprint to pay $5.50 in stock for each Virgin Mobile share. Sprint already owned 13.1 percent of Virgin Mobile, which uses Sprint's network to offer service.

The offer is a 31 percent premium to Virgin Mobile's closing share price Monday of $4.21. In early trading Tuesday, the shares rose $1.05, or 25 percent, to $5.26.

Virgin Mobile resells access to Sprint's wireless network for people who lack the credit or income to sign monthly plans. It has 5.2 million subscribers who pay an average of $20 per month. Sprint has 49.1 million subscribers, including those using the network through wholesalers like Virgin Mobile.

The deal reinforces this year's main trend in wireless: The top two carriers, Verizon Wireless and AT&T Inc., are grabbing the high-value contract customers, while Nos. 3 and 4, Sprint and T-Mobile USA, are left to compete for prepaying customers with smaller upstarts like MetroPCS Communications Inc. and Leap Wireless International Inc.

Sprint roiled the industry by introducing a $50-per-month prepaid unlimited plan in January under its Boost brand. That made for an awkward relationship with its customer Virgin Mobile, which had a $80-a-month