A light-triggered behavioural switch in cyanobacterial motility

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Cyanobacteria are phototrophic prokaryotes performing plant-like photosynthesis. They are the ancestors of all plant and algae chloroplasts. Besides using them as model organisms for oxygenic photosynthesis there is an increasing interest in the public because of their potential for biotechnological applications. Like for all phototrophs light sensing is of crucial importance for cyanobacteria to acclimate to different environmental conditions. Accordingly, they have to choose the optimal light conditions to absorb photons for photosynthetic energy supply and at the same time to avoid high- and UV-light stress. This requires efficient acclimation strategies for grow under different light conditions. The photoreceptor Cph2 from Synechocystis sp. PCC 6803 senses four different colours by harbouring two distinct photosensory modules. The light signals are transduced to the enzymatically active effector modules which synthesize or degrade the second messenger molecule c-di-GMP. Depending on the concentration of this molecule cyanobacteria decide whether to move towards a light source or to induce biofilm formation. We have demonstrated that the multi-domain sensor Cph2 alters the c-di-GMP level in cyanobacterial cells under blue light, thus inhibiting phototaxis towards these unfavourable light conditions. Though the structure of the red/far-red absorbing phytochrome module has been solved, we still do not understand the biological function of this part of the light switch. In the project proposal we want to elucidate how the different light signals sensed by Cph2 are integrated and transmitted to downstream components of a c-di-GMP related signal transduction chain that controls cyanobacterial motility. Further we want to identify the unknown c-di-GMP effector proteins and to analyze their roles in regulating motility. We will use spectroscopic analysis of recombinant proteins, protein interaction studies, optogenetic tools and phototaxis tests with various mutant strains of regulatory and structural elements to unravel the underlying molecular mechanism of Cph2 based signaling processes in the cyanobacterial model strain.