OSCILLATING SIGNALS IN THE SENSORY PATHWAY OF HALOBACTERIA INDUCED BYPERIODIC LIGHT STIMULI

The swimming behaviour of Halobacterium salinarium can be modulated by light. Changes of the light intensity that induce reversals of the sw imming direction are called repellent stimuli, those that suppress rev ersals, which otherwise would occur spontaneously from time to time, a re called attractant stimuli. Bacteria were stimulated by periodic pul se-like stimuli, and the frequency of induced reversals was recorded. Stimulation with a period length between 16 and 6.5 s let the cells re verse periodically with the frequency of the external force. After the stimulation had been stopped, the cells continued to reverse periodic ally for 3 to 9 periods which, however, switched to a value of about 6 to 8 s, independent of the frequency of preceding stimulation. This e ndogeneous oscillation was most distinct when the stimulation period e ither equalled the endogeneous period or was twice or half of its leng th. During the endogeneous oscillation, the responsiveness to an attra ctant stimulus showed a pronounced phase-dependence. These results poi nt to the oscillation of a signal in the sensory pathway which, differ ent from our former assumption, seems to be not self-sustained but has to be set going by external stimulation.