Fidelity's ProBuild investment stumbles -- filing

* Fidelity parent put up $345 mln to back ProBuild

* Invested in building supply co just before housing slump

* Continued acquisitions amid downturn

By Ross Kerber and Aaron Pressman

BOSTON, Nov 5 (Reuters) - Fidelity Investments' controlling Johnson family may run one of the world's largest financial companies, but when it comes to their own investments, the timing has not always worked out so well.

Fidelity's parent, FMR LLC, has spent $345 million over the past six months to cover losses at ProBuild, one of the largest U.S. building materials suppliers, which it built up at the height of the real estate bubble.

Under a recapitalization plan adopted in May, Fidelity could be on the hook for another $105 million through January 2010, according a confidential prospectus for a recent Fidelity debt offering obtained by Reuters.

Losses have been mounting at Denver-based ProBuild, which operates 470 building supply stores and lumber yards.

Revenue this year is expected to total just $3 billion, half of the chain's total revenue in 2006, when Fidelity assembled the company.