Prudential Financial Inc. (PRU) said Wednesday after the market close that Nippon Life Insurance Co. has signed an agreement to purchase a $500 million 10-year exchangeable surplus note.

Prudential will pay interest at an annual rate of 5.36. The note can be exchanged at Nippon's option for common stock after five years to give Nippon up to a 9.9% stake in Prudential, the press release said. The transaction is expected to close Friday.

Nippon, based in Osaka, Japan, is a mutual insurer that offers annuities, life, medical, and nursing coverage and had year-end 2008 total assets of $467 billion, and 2008 revenue of $67 billion.

Nippon owns a 7% stake in Principal Financial Group (PFG) and has had an ongoing business relationship with Principal for more than 20 years. It is Principal's largest shareholder.

Shares of Prudential closed up 7.4% Wednesday at $54.63, but traded down 3.8% at $52.54 after the market closed.

-By Lavonne Kuykendall, Dow Jones Newswires; (312) 750 4141; lavonne.kuykendall@dowjones.com