The molecular basis of sensing and responding to light in microorganisms

Photon absorption in biological signal transfer is mediated by a limited number of photoreceptor families, each characterised by binding of chromophore from a particular class of chemical compounds. Most photosensors become activated upon light-induced E/Z (i.e., trans/cis) isomerization of a double bond in their chromophore. This change in configuration of the chromophore subsequently must be translated into a change in the conformation of the photosensor protein, and transmitted to the downstream partner in the signal transduction chain. Particularly in archaea the molecular mechanism of signal transfer from the photosensor all the way to its target, i.e., the motility machinery, specific promoters and/or specific enzymes, is well understood for selected examples. In most of these, this flow of information makes use of a mechanism that is based on the so-called 'two-component paradigm'. Best know among these are the light-induced behavioural responses in Halobacterium salinarum, i.e., attraction by green- and repulsion by blue light. Regarding eukaryotic microorganisms our understanding of light-induced signal transfer, beyond the photoreceptor proteins, is restricted. This is due to their much more complex motility apparatus, the involvement of various secondary messengers and their compartmentalisation. The latter may require translocation of transcriptional activators to the nucleus and may form the basis of the sensing of the direction of the light. For a limited number of photoreceptor proteins we begin to understand the intra-molecular transition required to bring about the change in conformation of the protein that initiates signal transfer, i.e., the structure of the so-called 'signalling state'. This insight is most advanced in the photoactive yellow protein from Ectothiorhodospira halophila, a photoreceptor initiating a repellent response upon blue light excitation. In some well-established examples of sensor proteins involved in the transmission of chemical signals, formation of the signalling state appears to be just a shift in the equilibrium between two states that both are already present in the absence of signals. In photoreceptor proteins, however, this situation appears to be much more complex.