Kinetically resolved states of the Halobacterium halobium flagellar motor switch and modulation of the switch by sensory rhodopsin I

Abstract

Spontaneous switching of the rotation sense of the flagellar motor of the archaebacterium Halobacterium halobium and modulation of the switch by attractant and repellent photostimuli were analyzed by using a computerized cell-tracking system with 67-ms resolution coupled to electronic shutters. The data fit a three-state model of the switch, in which a Poisson process governs the transition from state N (nonreversing) to state R (reversing). After a reversal, the switch returns to state N, passing through an intermediate state I (inactive), which produces a ca. 2-s period of low reversal frequency before the state N Poisson rate is restored. The stochastic nature of the H. halobium switch reveals a close similarity to Escherichia coli flagellar motor properties as elucidated previously. Sensory modulation of the switch by both photoattractant and photorepellent signals can be interpreted in terms of modulation of the single forward rate constant of the N to R transition. Insight into the mechanism of modulation by the phototaxis receptor sensory rhodopsin I (SR-I) was gained by increasing the lifetime of the principal photointermediate of the SR-I photochemical reaction cycle, S373, by replacing the native chromophore, all-trans-retinal, with the acyclic analog, 3,7,11-trimethyl-2,4,6,8-dodecapentaenal. Flash photolysis of analog-containing cells revealed an eightfold decrease in the rate of thermal decay of S373, and behavioral analysis showed longer periods of reversal suppression than that of cells with the native chromophore over similar ranges of illumination intensities. This indicates that attractant signaling is governed by the lifetime of the S373 intermediate rather than by the frequency of photocycling. In this sense, SR-I is similar to rhodopsin, whose function depends on an active photoproduct (Meta-II).

Damped oscillations in photosensory transduction of Halobacterium salinarium induced by repellent light stimuli

“…1A), in accordance with previous results (10,15). Almost all cells reversed their swimming direction 2 to…”

“…However, this response fails to occur if the stimulus is applied during the first half-second after a reversal has taken place, during the so-called refractory period. Cells regain responsiveness within the following 2 s (10,15).…”

Rotation and switching of the flagellar motor assembly in Halobacterium halobium

“…Given the exponential decay of the frequency distribution, we suggested spontaneous motor switching to be a stochastic event and introduced a four-state model of the flagellar motor switch sufficient to fit the experimental data quantitatively (21). Both the exponential decay and the interpretation of motor switching as a stochastic event were confirmed by McCain et al, who in addition showed that the length of a given swimming interval is independent of the length of the preceding one (24). In this context, it is of interest that a four-state model has also been proposed for the motor switch in E. coli (12) and that threshold crossings of a regulatory substance subjected to statistical fluctuations have been excluded from causing spontaneous motor switching (5).…”

Photosensory Adaptation in Aneural Organisms*

“…The exponential decay of the frequency distribution for flagellar reversal was taken as evidence that the spontaneous motor switching is a stochastic rather than an oscillator-mediated event (Marwan et al, 1991). The interval distribution could be fitted either to a symmetric four-state model (Marwan and Oesterhelt, 1987) or an asymmetric three-state model (McCain et al, 1987) of the flagellar switch. Such models have also been proposed for the chemotactic behavior of Salmonella typhimurium (Kuo and Koshland, 1989).…”