Bacterial photosensory proteins: regulatory functions and optogenetic applications

Three classes of light-sensory regulatory proteins, which have been identified in genomes of numerous phototrophic and nonphotosynthetic bacteria, are discussed: the UVA/blue light sensitive BLUF and LOV domain-containing proteins and red/far-red light-sensitive phytochromes. Light perception by these chromoproteins is provided by the flavin or bilin (in phytochromes) chromophores binding to their photosensory domains. Bacterial photoreceptors also contain a variety of effector domains with enzymatic DNA-binding and other functions, which compose modular light-switchable systems. In recent years, progress was achieved in uncovering the photoactivation mechanisms of such systems. Based on the chromophore phototransformation-induced changes in the domain structures, these mechanisms cause the biochemical signal cascades which can control the light-dependent physiological responses of the cells. The new information obtained is important not only for understanding the fundamental mechanisms of light perception and signal transduction by bacterial photosensory proteins but also as a basis for designing photo-switchable enzymes and light-inducible gene expression systems, which may be used in optogenetics, a new field in cell biology and biotechnology. The presents review is focused on the structural aspects of signal transduction in light-activated bacterial photoreceptors, on their regulatory functions, and on some recent advances in using LOV and BLUF photosensors in optogenetics for the regulation of biological processes.