Real estate developer St. Joe Co., Florida's second-largest private landowner, has agreed to let its largest shareholder, Bruce Berkowitz's Fairholme Capital of Miami, buy up to 50 percent of its shares. The move secures the grip on St. Joe by Fairholme, which already owns 29 percent of the company. Shares in St. Joe, which is based in Watersound on the Florida Panhandle, have lost 30 percent since the end of April. Berkowitz has been increasingly drawn into St. Joe's turnaround efforts after hedge fund manager David Einhorn blasted the company as overvalued last October. St. Joe owns about 574,000 acres in northwest Florida. Its development efforts have struggled since the real estate bubble burst in 2005, when its shares peaked at over $80. In June, St. Joe disclosed that the Securities and Exchange Commission was investigating whether the company and its past and present officers and directors had violated antifraud provisions of federal securities laws and whether Fairholme had met its disclosure requirements as a beneficial owner of more than 5 percent of St. Joe shares.