* Move makes Cerberus Commerzbank's 2nd-biggest shareholder
* Other major shareholders are the German govt, BlackRock
* Comes as bank focuses on restructuring (Updates with Cerberus declining comment, share price, background, government comment)
FRANKFURT, July 26 (Reuters) - Commerzbank said on Wednesday that U.S. buyout fund Cerberus has built a stake controlling 5.01 percent of the German bank's voting rights, as of July 25.
Cerberus now ranks as Commerzbank's second-largest shareholder, behind the German government with a 15.6 percent stake and marginally ahead of BlackRock with a 5.0 percent stake, according to Thomson Reuters data.
The value of the stake is about 678 million euros ($789 million), based on current market capitalization.
The German government welcomed the Cerberus investment. "Rising investor interest is positive for Commerzbank and its share price," said a spokeswoman for the German Finance Ministry. "We haven't changed our position on Commerzbank." The guiding principle, she said, was: "We want a good financial result for taxpayers."
Representatives for Commerzbank and Cerberus declined to comment.
Cerberus has made acquisitions in European financial institutions. It owns 52 percent of Austrian bank BAWAG , which is making preparations for an initial public share offering that could value it at up to 5 billion euros.
BAWAG said this month it had bought German regional lender Suedwestbank for an undisclosed price to expand its network in western Europe.
Shares in Commerzbank, Germany's second-biggest bank, extended gains on the news, trading 2.1 percent higher by 1414 GMT, putting them among the top gainers on Germany's blue-chip DAX index, which was up 0.4 percent.
In recent months, Commerzbank has been implementing a restructuring program, focusing on digitizing its back office, cutting staff, and growing its retail customer base.
The bank has warned that it would post a loss in the second quarter after market weakness and restructuring costs. It is scheduled to report earnings on Aug. 2.
($1 = 0.8592 euros) (Reporting by Tom Sims, Alexander Huebner, Gernot Heller; Editing by Maria Sheahan and Adrian Croft)