Antonie van Leeuwenhoek 65:331-347, 1994. 331 @ 1994 KluwerAcademicPublishers. Printed in the Netherlands. Photobiology of Bacteria K.J. Hellingwerf a, W. Crielaard 1, W.D. Hoff 1, H.C.R Matthijs 1,2, L.R. Mur 2 & B.J. van Rotterdam 1 Department of Microbiology; ~ E.C. Slater Institute; 2Amsterdam Research Institute of Substances in the Environ- ment, Nieuwe Achtergracht 127, 1018 WS Amsterdam, The Netherlands Key words: photoactive proteins, photoreceptors, chromophores, energy transduction, light signalling, phototaxis, gene expression Abstract The field of photobiology is concerned with the interactions between light and living matter. For Bacteria this interaction serves three recognisable physiological functions: provision of energy, protection against excess radiation and signalling (for motility and gene expression). The chemical structure of the primary light-absorbing components in biology (the chromophores of photoactive proteins) is surprisingly simple: tetrapyrroles, polyenes and derivatised aromats are the most abundant ones. The same is true for the photochemistry that is catalysed by these chromophores: this is limited to light-induced exciton- or electron-transfer and photoisomerization. The apoproteins surrounding the chromophores provide them with the required specificity to function in various aspects of photosynthesis, photorepair, photoprotection and photosignalling. Particularly in photosynthesis several of these processes have been resolved in great detail, for others at best only a physiological description can be given. In this contribution we discuss selected examples from various parts of the field of photobiology of Bacteria. Most examples have been taken from the purple bacteria and the cyanobacteria, with special emphasis on recently characterised signalling photoreceptors in Ectothiorhodospira halophila and in Fremyella diplosiphon. 1. Introduction Classification of photobiological processes The aim of studies in photobiology is the characteriza- tion of the interactions between light and living matter (Song et al. 1991; Clayton 1977). Light plays var- ious roles in the life of a microorganism (see Table 1). First, and of far most importance, light supplies the organisms that have learned to live the phototroph- ic mode of life with free energy for maintenance and growth. For this purpose a large array of antenna com- plexes and reaction centers has evolved during evolu- tion. As a result, two mechanistically very different types of photosynthesis have come into being: (bac- terio)chlorophyll based- and retinal-based photosyn- thesis (for a comparison of these two forms of pho- tosynthesis: see Hellingwerf et al. 1994). Chlorophyll based-photosynthesis itself developed into two clear- ly distinguishable forms: anoxygenic- and oxygenic photosynthesis. Light, however, is not only useful; it is dangerous too. Upon absorption of a visible photon, for instance, in a photoactive protein the immediate surrounding of the light-absorbing molecule (the chromophore) heats up some 2000 C between the picosecond and microsec- ond time domains (Li & Champion 1994). This notion makes it easy to imagine that photoactive proteins are prone to damage. However, even prior to this heat- ing, other types of damage can occur. The excited singlet- and triplet-state of many chromophores can react with molecules like oxygen, to give rise to the highly reactive superoxides. These molecules may sub- sequently covalently modify proteins in a cell at ran- dom. In addition, radiation from the (near)UV-part of the visible spectrum may cause damaging photo-