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The attractant effect of green light and the repellent effect of blue light on Halobacterium halobium were studied. It was found that addition of CN− and dicyclohexylcarbodiimide, which block the redox chain and H+-ATPase, respectively, increased both the amplitude of light-dependent changes of membrane potential (∆ψ), monitored by the distribution of tetraphenyl-phosphonium, and the sensitivity of the green-light taxis. A direct proportionality between ∆ψ and the green-light sensitivity was revealed. The sensitivity of the green-light taxis was decreased by glucose, histidine and Na+/K+ gradient, i.e. factors reducing the light-dependent changes of protonic potential. These factors did not affect the blue-light taxis. The uncoupler carbonyl cyanide m-chlorophenylhydrazone appears to repel H. halobium cells. These data are in agreement with the assumption that the green-light taxis is governed by a sensing of protonic potential, whereas the blue-light taxis is mediated by a specific photoreceptor.
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