Phototaxis and the Second Sensory Pigment in Halobacterium halobium

Abstract

Halobacterium halobium is an archaebacterium well known for its unique light energy-transducing apparatus, bacteriorhodopsin. Usually the bacterium is energized by respiration. When the cells grow so dense that the supply of oxygen becomes insufficient, the cells produce a large amount of bacteriorhodopsin and utilize the energy of visible light. Therefore, bacteriorhodopsin appears to be an emergency apparatus [1]. The cell growing in such a dense culture is attracted to visible light. The cells accumulate at a spot of green-yellow light. On the other hand, UV and violet light are avoided by the cells [2]. The behavioral response of the cells to light is similar to that of enteric bacteria to chemical substances [3], namely, the frequency of the change in swimming direction is increased or decreased when a repellent stimulus or an attractant one is introduced to the cell, respectively. The difference lies in the way the change of swimming direction is brought about. Halobacterium halobium has flagella in both poles of the cell and swims indifferently in either direction. Therefore, the bacterium simply reverses the swimming direction by reversing the direction of flagellar rotation when repellent light is applied (step-up Photophobic response) or attractant light is taken away (step-down Photophobic response). The photoattractant and the photorepellent response are controlled by different photosystems. The photorepellent system that shows maximal response of the cells to 370nm light was named photosystem 370 (PS370) [2].