EARLIER THIS YEAR, SIRNA THERAPEUTICS ANNOUNCED it was moving its corporate headquarters from Boulder, Colo., to San Francisco — one more in the long line of biotechnology firms to put down roots in the region. From a real-estate perspective, homegrown and transplanted companies together have transformed the fabled Bay area into the largest biotech community in the country, occupying 16 million square feet. And demand for laboratory space, from San Francisco to Palo Alto, shows no sign of slowing, as the proximity of Genentech and first-rate universities beckons other research firms.