Sensory rhodopsin-controlled release of the switch factor fumarate in Halobacterium salinarium

Halobacterium salinarium responds to blue light by reversing its swimming direction. Fumarate has been proposed as one of the molecular components of this sensory system and is involved in the switching process of the flagellar motor. In order to obtain chemical proof for this role of fumarate, cells were stimulated with a pulse of blue light and lysed by rapid mixing with distilled water. The lysate contained fumarate in free and bound form, which were separated by ultrafiltration. The fumarate concentration in the low-molecular-mass fraction (< 5 kDa) of the lysate was assayed enzymatically and a light-induced increase was observed. Additionally, the total cellular fumarate content decreased in response to light, indicating that fumarate was released from a cellular pool rather than being formed by de novo synthesis. The light-induced release was not detected in a mutant defective in sensory rhodopsin-I and -II. Therefore it is concluded that photoreceptor activation rather than a direct effect of light on the activity of metabolic enzymes causes fumarate release. For each photoactivated sensory rhodopsin-II molecule at least 350 molecules of fumarate were liberated demonstrating efficient amplification. The rate of light-induced fumarate release is at least 10-times faster than the fumarate turnover number of the citric acid cycle which was estimated as approximately 4300 per cell and second. Therefore this metabolic process is not expected to be part of the signal transduction chain in the halobacterial cell.