| Abstract: | Halobacteria detect changes in light intensity by retinal proteins, the number and identity of which are not yet unequivocally established. The sensory receptors are different from those for light energy conversion. The cells having no preferred swimming direction spontaneously reverse about every 10 s. An oscillator model has been proposed to explain this periodicity. Depending on wavelength and sign, a stimulus leads either to one prolonged interval between two reversals, the attractant response, or to a shortened interval, the repellent response. Sensory signals generated by stimulation of P-565 and of P-370 are integrated at a common link. Signals from other receptors may be processed by separate links. The nature of the sensory signals is not known, but the membrane potential can be excluded as a candidate. On the basis of the oscillator hypothesis the output signals of the integration links act on the oscillator and thus shift the time at which it triggers a reversal of the flagellar motor. Experiments indicate that cGMP and calcium play antagonistic roles in the oscillatory activity. Reversible methylation of specific membrane proteins influences the time during which successive signals are integrated. This reaction is assumed to terminate the lifetime of the excitatory signals and thus to allow the system to adapt. |
