Light is one of the most important signals providing critical information to biological systems. In addition to human vision, light controls movement, growth, differentiation and development, circadian clock, and a host of gene-expression responses in diverse organisms from prokaryotic microbes to eukaryotic plants and animals. The second conference of the new Gordon Conference series will focus on current research on photosensory receptors, covering topics of their structural, functional and mechanistic implications. Rapid progress has been and is being made on several long-standing fundamental questions about the mechanisms of photon energy capture and transduction in different organisms including plants, animals, eukaryotic and prokaryotic microbes. Researchers who investigate photosensory receptors in such very different living systems are confronting many of the same challenges in elucidating the sensory transduction mechanisms and pathways involved in photosensory processes. Recognizing the common challenges and the need for a dedicated meeting to bring together this group of researchers, an exploratory meeting under the same title as this Conference was organized in Barga, Italy, 2000 (co-organized and chaired by John Spudich and Winslow Briggs). Following the success of that meeting, the charter Gordon Conference was held, also in Barga, Italy, from April 30-May 5, 2002, under the sponsorship of Gordon Research Conferences. The 2002 meeting provided a very much-needed opportunity to exchange information and ideas and to become aware of some of the diverse tools available for the study of photosensory biology.

Since the first meeting two years ago, several new classes of photosensory receptors have been discovered and characterized. These include: phytochromes in prokaryotes homologous to those in higher plants, the flavoprotein cryptochromes in plants, animals and microbes, phototropins in different organisms, photoactivated adenylyl cyclase (PAC) as Euglena's blue light receptor, retinal proteins homologous to archaeal rhodopsins, photoactive yellow proteins (PYPs) in photosynthetic bacteria, and others. Genomic information is becoming powerful tool to clarify their evolution and distribution in nature. Their mechanisms of activation and signal transduction are being unraveled at a rapid pace. This Conference is expected to provide a forum for stimulating and intense discussion of mechanistic insights and general principles emerging from the cutting edge research in photosensory signal transduction.