The heads of Tesla Motors Inc. and Toyota Motors Corp. surprised the auto world Thursday by announcing a partnership to develop and build electric cars at a recently shuttered auto plant in the San Francisco Bay area. Akio Toyoda, CEO of the world's largest automaker, said Toyota will invest $50 million in Tesla when the company begins selling stock to the public, and Tesla CEO Elon Musk said his company will purchase the New United Motor Manufacturing Inc. factory, known as Nummi, in Fremont where the Model S electric sedan will be built. Analysts said the partnership enhances the credibility of Tesla, a Silicon Valley start-up just a few years ago, and boosts Toyota's image after being fined a record $16.4 million for its slow response to an accelerator pedal recall.