Profile of Susan S. Golden.

Susan Golden did not set out to become an expert in biological clocks, the internal timepieces that keep life on Earth adjusted to a 24-hour cycle. Instead, Golden, elected in 2010 to the National Academy of Sciences, wanted to identify the genes that underpin photosynthesis. However, her focus changed in 1986 with the discovery of biological clocks in cyanobacteria (1). Because cyanobacteria are among Earth’s earliest living organisms, the discovery made clear that biological clocks are evolutionarily ancient. Golden had been studying photosynthesis in cyanobacteria since graduate school. Her reason was simple: Cyanobacteria are single-celled, and thus they are much easier to manipulate in a laboratory than plants. With her expertise in cyanobacteria, Golden found herself well-positioned to identify the genes and proteins that make the clock tick. “Cyanobacteria are present in niches and habitats all over the planet,” she says. “They’re very good at adapting to their habitat and that adaptation involves doing things at the right time.” Their timekeeping ability, she adds, helps cyanobacteria generate about 30% of atmospheric oxygen. Over the years, Golden and her collaborators have revealed that the clock in cyanobacteria works like a mechanical clock, complete with oscillators, gears, and hands. In her recent research, including her Inaugural Article, Golden has shown how clock proteins interact to synchronize the internal clock with the external 24-hour cycle (2, 3). To Golden’s own surprise, this recent line of inquiry has taken her back to her roots in photosynthesis. Her findings show that cyanobacteria don’t use sensory photoreceptors, proteins that convey lightrelated information into the body in mammals, to set their clocks. Instead, cyanobacteria integrate the ability to tell time with their photosynthetic apparatus. In retrospect, says Golden, this finding makes sense: In the prokaryote’s world, she says, “If photosynthesis is running, the lights are on. It’s daytime. If photosynthesis is not running, it’s dark.”