PUMA
Istituto di Biofisica     
Petracchi D., Lucia S., Cercignani G. New trends in photobiology : Photobehaviour of Halobacterium halobium: proposed models for signal transduction and motor switching. In: Journal of Photochemistry and Photobiology B-Biology, vol. 24 (2) pp. 75 - 99. Elsevier, 1994.
 
 
Abstract
(English)
The swimming behaviour of Halobacterium halobium is influenced by variations in light intensity and wavelength. These stimuli are sensed through two retinal-containing proteins: sensory rhodopsin I and II (SR-I and SR-II), each displaying distinct spectra and photocycles. Light as an energy source is used by H. halobium through two other retinal-containing membrane proteins, acting as photon-driven ionic pumps, bacteriorhodopsin and halorhodopsin. In their ground states, SR-I and SR-II have an absorption maximum at 587 and 487 nm respectively. They have been shown to function as photoreceptors that signal, by a chemical transduction chain, to the motor switch controlling the direction of rotation of a flagellar bundle at the cell pole. Changes from clockwise to counterclockwise rotation or vice versa produce reversals in the swimming direction of H. halobium. Phototaxis occurs because the time interval between reversals is altered by response to light stimuli. SR-I acts as a receptor for attractant (red—orange) light stimuli and displays photochromic properties: when absorption in this wavelength range increases, reversals become less frequent and the pigment is transformed into a blue-absorbing species (SR373). This intermediate of the SR-I photocycle appears to act both as a signalling state for the reversal-suppressing transduction chain and as a receptor for repellent (blue) light stimuli; these are effective in eliciting more frequent reversals when delivered against a red—orange background. SR-II, also called phoborhodopsin, only senses blue—green light (repellent) stimuli, producing an increased frequency of reversals. This sensor—effector system has been subjected to various types of analysis and constitutes the second best known example of this kind in prokaryotes, the first being chemotaxis in enterobacteria. Differences and similarities between these two natural models are described. Several mathematical models have been proposed to interpret the mechanisms underlying taxis phenomena. These are typically kinetic schemes for the transduction chain, which are partly hypothetical in H. halobium, due to the limited amount of convincing data on the nature of the (chemical) signal reaching the motor switch. Basically, two types of model have been suggested to account for the time interval distribution of reversals in the swimming behaviour of H. halobium: in the first (of which two variants have been proposed), the stochastic properties of the putative transduction chain signalling to the motor switch are involved; in the second, the hypothesis that an endogeneous oscillator is responsible for the motor switch control is maintained. Structural and biochemical data relevant for a model treatment of the processes occurring at the transduction chain are reported. The available behavioural data in H. halobium are also reported and discussed in detail, comparing the experimental approaches and the different interpretations. The implications of the proposed kinetic models are discussed.
DOI: 10.1016/1011-1344(94)07009-1
Subject Photomovements
Phototaxis
Photoreception
Sensory transduction
Halobacterium halobium
Escherichia coli
Photocycle
Bacterial rhodopsins
Sensory rhodopsins
Phoborhodopsin
Methyl-accepting proteins
Mathematical models of taxis


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