Role of the response oscillator in inverse responses of Halobacterium halobium to weak light stimuli

Abstract

(3,4,15). According to the wavelengths of maximal quantum efficiency the two sensory photosystems were called PS 565 and PS 370 (3). Recently an additional pigment which absorbs maximally around 475 nm was found to contribute to the sensitivity in the blue-UV range (17).The magnitude of the behavioral responses usually depends directly on the stimulus strength, i.e., on the increase or decrease of the fluence rate (4). Under certain conditions, however, weak attractant stimuli can lead to repellent responses and weak repellent stimuli cause attractant responses. In the present paper we describe such inverse responses. Furthermore, we discuss these phenomena in relation to a cellular oscillator for which we previously proposed a role in response regulation and sensory control in halobacteria (10, 11). MATERIALS AND METHODSBacterial strains and cultivation. Cells of Halobacterium halobium with different interval lengths between spontaneous reversals were used. Strain WS (resembles the wild type in pigment content) had the shortest intervals; R1L3 (lacks most of the carotenoids) had intervals of medium length; ET15-7 (lacks bacteriorhodopsin and carotenoids and does not respond to stimuli in the yellow-green range) had the longest intervals (5); Flx3 lacks the light energy-converting * Corresponding author. pigments bacteriorhodopsin and halorhodopsin (13). Cultivation was done as described earlier (5). Bacteria from the early stationary phase were used.It was possible to obtain cells with extremely short or long intervals from these strains by further selection. Single colonies were isolated; after growth in peptone medium, the cultures were tested for the length of spontaneous reversal intervals. The cultures with the longest intervals were directly used for experiments; cultures with short intervals were further selected for high motility on soft agar plates (15).Experimental procedure. Single cells in a population were observed through a phase-contrast microscope connected to an infrared-sensitive video system. At given delay times after a spontaneous reversal had been observed, stimuli (stepwise increase or decrease of light) were applied through the objective by an incident light illuminator.The interval length between reversals before and after the stimulus was measured with an electronic stopwatch. The response is defined as an attractant response when the interval is longer, and as a repellent response when it is shorter, than the average spontaneous interval.The wavelength of stimulating light was either 565 or 370 nm. If not otherwise noted, background light for observation was white light (>350 nm, 250 W m-2). Further details of the experimental prodecure, including light measurement, were described elsewhere (5). The temperature was 22 to 24°C. RESULTSIn most strains of H. halobium the mean interval length between spontaneous reversals is about 10 to 12 s. Some strains we used showed longer or shorter mean interval lengths. By isolation of single cells out of these populations it was possible to get r...

Effects of sequential stimuli on Halobacterium salinarium photobehavior

“…Under particular experimental conditions, however, the occurrence of so-called inverse or paradoxical responses (i.e., photophobic responses to attractant stimuli or depression of reversals after a repellent stimulus) has been reported. Hildebrand and Schimz (1987) claimed that inverse responses could occur within several seconds after a spontaneous reversal for both attractant and repellent stimuli. However, their data were obtained by measuring the response through the average interval after the stimulus.…”

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

“…Some * Corresponding author. of these effects have been attributed to photoreceptor reactions (e.g., the photochromic states of SR-I and simultaneous stimulation of SR-I and SR-Il [23,31,32,35]), and some have been attributed to an endogenous oscillator postulated to control the flagellar motor switch (11,14,15).…”

“…Studies of the output behavioral response have shown complexities of reversal frequency modulation by photostimuli, such as dependence of the magnitude of the response on time after the previous reversal (14) and inversions of the sign of the response depending on background light, strain, stimulus intensity and time after the previous reversal (11,23,31,32,35). Some * Corresponding author.…”

A rapid population method for action spectra applied to Halobacterium halobium

“…This light intensity is comparable with or lower than the intensities used by other workers (3,11,12,22,23).…”