Packaging manufacturer Rock-Tenn Co. is buying competitor Smurfit-Stone Container Corp. for $3.5 billion in cash and stock, the companies announced Sunday.

The combined company will have annual sales of more than $9 billion, they said, making it one of the largest North American paperboard companies.

The deal values Smurfit-Stone's common shares at $35, a 27-percent premium over Friday's closing price of $27.52.

Both companies' boards agreed to the half-stock, half-cash deal. Rock-Tenn, based in Norcross, Ga., will pay 0.30605 share of its own stock, plus $17.50 in cash, for each share of Smurfit-Stone.

After the deal, Rock-Tenn's current shareholders will own about 56 percent of the combined company's shares. Smurfit-Stone's current shareholders will own the remaining 44 percent.

Smurfit-Stone, based in Chicago, emerged from Chapter 11 bankruptcy protection seven months ago.

Rock-Tenn will assume $700 million in debt and $1.1 billion in pension liabilities from Smurfit-Stone as part of the deal.

The deal is expected to close in the second quarter of 2011. It still must be approved by shareholders of both companies and by regulators.