Effects of cGMP,calcium and reversible methylation on sensory signal processing in halobacteria

Cyclic GMP and Ca²⁺ change the following parameters of sensory behavior of Halobacterium halobium in opposite directions: the interval between two spontaneous reversals of the swimming direction, the phases of increasing and decreasing responsiveness to light stimuli during an interval and the length of the refractory period. Inhibition of the methylation of membrane proteins by blocking the methyltransferase increases the attractant response and prolongs the time during which successive stimuli are integrated. A decrease of the external Ca²⁺ concentration, which activates methylation, has the opposite effect. Inhibition of methylation also prevents the entrainment of the reversal frequency by rhythmic repellent stimuli. The result suggest that cGMP and Ca²⁺ may be antagonistic components of an oscillator, which generates the autonomous rhythm of flagellar reversals, while methylation determines the life-time of sensory signals at an integration link prior to the oscillator, and thereby allows adaptation. Ca²⁺ is thought to act on the methylation system in a feedback loop.