PHOTORESPONSES IN RHODOBACTER-SPHAEROIDES - ROLE OF PHOTOSYNTHETIC ELECTRON-TRANSPORT

Rhodobacter sphaeroides responds to a decrease in light intensity by a transient stop followed by adaptation, There is no measurable respons e to increases in light intensity, We confirmed that photosynthetic el ectron transport is essential for a photoresponse, as (i) inhibitors o f photosynthetic electron transport inhibit photoresponses, (ii) elect ron transport to oxidases in the presence of oxygen reduces the photor esponse, and (iii) the magnitude of the response is dependent on the p hotopigment content of the cells, The photoresponses of cells grown in high light, which have lower concentrations of light-harvesting photo pigment and reaction centers, saturated at much higher light intensiti es than the photoresponses of cells grown in low light, which have hig h concentrations of light harvesting pigments and reaction centers, We examined whether the primary sensory signal from the photosynthetic e lectron transport chain was a change in the electrochemical proton gra dient or a change in the rate of electron transport itself (probably r eflecting redox sensing), R. sphaeroides showed no response to the add ition of the proton ionophore carbonyl cyanide 4-trifluoromethoxypheny lhydrazone, which decreased the electrochemical proton gradient, altho ugh a behavioral response was seen to a reduction in light intensity t hat caused an equivalent reduction in proton gradient, These results s trongly suggest that (i) the photosynthetic apparatus is the primary p hotoreceptor, (ii) the primary signal is generated by a change in the rate of electron transport, (iii) the change in the electrochemical pr oton gradient is not the primary photosensory signal, and (iv) stimuli affecting electron transport rates integrate via the electron transpo rt chain.