UPDATE 1-German building materials firm Knauf offers to buy USG

(Adds more details on the offer; Updates shares)

March 26 (Reuters) - USG Corp's second biggest shareholder, Germany-based Gebr Knauf KG, has offered to buy the company in a deal that values the buildings products maker at $6 billion.

Gebr Knauf, which already owns a 10.5 percent stake in the company, has offered $42 per share for the remaining stake, a premium of 25 percent to the stock's Friday closing.

Warren Buffet's Berkshire Hathaway Inc, which has a stake of about 31 percent in USG, has offered a six-month option to sell all its shares in the company to Knauf as long as its offer for USG is for $42 per share or more. Berkshire is asking for an option purchase price of $2 per share.

The option provides a sweetener for Buffett to shed a profitable investment that originated in late 2008, at the height of the housing crisis, when Berkshire and Canadas Fairfax Financial Holdings Ltd bought $400 million of USGs debt.

The $2 per share cost of the option would provide Berkshire about $86.8 million upfront, based on its 43.39 million share USG stake.

Berkshire would keep that money if Knauf proved unable to buy USG. (Reporting by Arunima Banerjee in Bengaluru; Editing by Anil D'Silva)