Cyanobacteria are able to move directly towards or away from a light source, a process called phototaxis. Recent studies have revealed that the spherical unicellular cyanobacterium Synechocystis sp. PCC 6803 exhibits a cell polarity in response to unidirectional illumination and that micro-optic properties of cyanobacterial cells are the basis of their directional light sensing. Further functional and physiological studies highlight a very complex control of cyanobacterial phototaxis by sensory proteins, histidine kinases and response regulators. Notably, PATAN domain response regulators appear to participate in directional control of phototaxis in the cyanobacterium Synechocystis sp. PCC 6803. In this review we explain the problem of directional light sensing at the small scale of bacteria and discuss our current understanding of signal transduction in cyanobacterial phototaxis.
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