SENSORY RHODOPSIN-CONTROLLED RELEASE OF THE SWITCH FACTOR FUMARATE INHALOBACTERIUM-SALINARIUM

Halobacterium salinarium responds to blue light by reversing its swimm ing direction. Fumarate has been proposed as one of the molecular comp onents of this sensory system and is involved in the switching process of the flagellar motor. In order to obtain chemical proof for this ro le of fumarate, cells were stimulated with a pulse of blue light and l ysed by rapid mixing with distilled water. The lysate contained fumara te in free and bound form, which were separated by ultrafiltration. Th e fumarate concentration in the low-molecular-mass fraction (<5 kDa) o f the lysate was assayed enzymatically and a light-induced increase wa s observed. Additionally, the total cellular fumarate content decrease d in response to light, indicating that fumarate was released from a c ellular pool rather than being formed by de novo synthesis. The light- induced release was not detected in a mutant defective in sensory rhod opsin-I and -II. Therefore it is concluded that photoreceptor activati on rather than a direct effect of light on the activity of metabolic e nzymes causes fumarate release. For each photoactivated sensory rhodop sin-II molecule at least 350 molecules of fumarate were liberated demo nstrating 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.