Signal formation in the halobacterial photophobic response mediated by a fourth retinal protein (P480).

Halobacterial cells swim forward by clockwise, and backward by counterclockwise, rotation of their flagella. The changes of direction of rotation occur statistically and can be quantitatively described by a four-state model of the motor. Stimulation of the cells with blue light induces the formation of a signal that causes the motor to switch the direction of rotation. The results of step-up and flash experiments led to a kinetic equation that describes the signal formation as a photocatalytic process. The stimulating blue light is sensed either by sensory rhodopsin in the presence of green background light or by protein P480, which has a maximum in the action spectrum around 480 nm. P480, but not sensory rhodopsin, is synthesized by the cells constitutively, and both pigments together allow the cells to find optimal conditions during aerobic and phototrophic growth. The work presented here was reported at the U.S.-Israel Binational Science Foundation Meeting in Jerusalem, March 1986.