Roles of phytochrome, PixJ, and photosynthesis in photophobotaxis of the filamentous cyanobacterium Phormidium lacuna

Cyanobacterium Phormidium lacuna filaments move from dark to illuminated areas by twitching motility. Time-lapse recordings demonstrated that this photophobotaxis response was based on random movements with movement reversion at the light-dark border. The filaments in the illuminated area form a biofilm attached to the surface. The wild-type and two mutants, pixJ and cphA , were investigated for photophobotaxis at diverse wavelengths and intensities. CphA is a cyanobacterial phytochrome; PixJ is a biliprotein with a methyl-accepting chemotaxis domain and is regarded as a phototaxis photoreceptor. The cphA mutant exhibits reduced biofilm surface binding. The pixJ mutant was characterized by greater photophobotaxis sensitivity. Thus, PixJ is a negative photophobotaxis regulator and does not serve as a light direction sensor. 3-(3,4-dichlorophenyl)1,1-dimethylurea (DCMU) blocks electron transfer in PS II. At concentrations of 100 and 1000 µM DCMU, photophobotaxis was inhibited, whereas motility was not significantly affected. This specific effect indicates PS II’s central role in light-direction sensing. It is argued that the intracellular concentrations of regular photoreceptors, including CphA or PixJ, are too small for a filament to sense rapid light intensity changes in very weak light. Three arguments, specific inhibition by DCMU, broad spectral sensitivity, and sensitivity against weak light, support photosynthesis pigments use as photophobotaxis sensors.