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Signalling and Signal transduction via a new class of photo receptors: the Xanthopsins Remco Kort, Andrea Haker, Mary K. Phillips Jones1, Daan van Aalten2, Klaas Hellingwerf and Wim Crielaard E.C. Slater Institute, Microbiology Section, University of Amsterdam BioCentrum Amsterdam, The Netherlands. 1 Department of Microbiology, University of Leeds, UK 2 Cold Spring Harbor Laboratory, Cold Spring Harbor, NY USA. 1.Introduction The photoactive yellow protein (PYP) is a new type of photosensory protein. It has been identified in the extremophilic photosynthetic purple-sulphur bacterium Ectothiorhodospira halophila. The chromophore of this photosensor has been identified to be 4-hydroxy-cinnamic acid, linked to the Cys-69 via a thiol-ester-bond. This is the first identification of this thiol-ester linked chromophore in a photoreceptor protein [Hoff et al., 1994; Baca et al., 1994]. In addition, PYP’s from Rhodospirillum salexigens and Chromatium salexigens [Koh et al., 1996] were purified and a large number of micro-organisms contain an immunologically cross-reacting protein [Hoff et al., 1994]. More-over, genes encoding PYP have been cloned, not only from E. halophila, but also from Rsp. salexigens [Kort et al., 1996] and Rhodobacter sphaeroides. These results indicate the existence of a new family of photoreceptor-proteins, here we report experiments) to further substantiate the universal mechanism of signalling and signal transduction via this new class of photoreceptors, which we propose to call the Xanthopsins.