Nikko Securities Co., a Japanese investment firm,...

Nikko Securities Co., a Japanese investment firm, has acquired a 20% stake in a holding partnership formed by Blackstone Group, a rapidly growing investment banking firm, the two concerns said. The deal formalizes a business relationship that began when Nikko invested $100 million in a Blackstone buyout fund last year. It gives Blackstone an infusion of capital and better access to the Japanese capital markets. It is the latest in a series of alliances that has seen Japanese investment firms buy stakes in smaller Wall Street investment boutiques.