Effects of sequential stimuli on Halobacterium salinarium photobehavior

Abstract

We analyzed the motor photoresponses of Halobacterium salinarium to different test stimuli applied after a first photophobic response produced by a step-down of red-orange light (prestimulus). We observed that pulses given with a suitable delay after the prestimulus produced unusual responses. Pulses of blue, green, or red-orange light, each eliciting no response when applied alone, produced a secondary photophobic response when applied several seconds after the prestimulus; the same occurred with a negative blue pulse (rapid shut-off and turning on of a blue light). Conversely, no secondary photophobic response was observed when the test stimulus was a step (a step-up for red-orange light, a step-down for blue light) of the same wavelength and intensity. When the delay was varied, different results were obtained with different wavelengths; red-orange pulses were typically effective in producing a secondary photophobic response, even with a delay of 2 s, whereas the response to a blue pulse was suppressed when the test stimulus was applied within 5 s after the prestimulus. The secondary photophobic response to pulses was abolished by reducing the intensity of the prestimulus without affecting the primary photophobic response. These results, some of which were previously reported in the literature as inverse effects, must be produced by a facilitating mechanism depending on the prestimulus itself, the occurrence of reversals being per se ineffective. The fact that red-orange test stimuli are facilitated even at the shortest delay, whereas those of different wavelengths become effective only after several seconds, suggests that the putative mechanism of the facilitating effect is specific for different signaling pathways.

Chemotaxis mediated by NarX–FrzCD chimeras and nonadapting repellent responses in Myxococcus xanthus

“…This mechanism for the biased movement of M. xanthus in a repellent gradient is consistent with all experimental results without the requirement of adaptation and without invoking spatial sensing. This is also somewhat reminiscent of the photophobic or light‐induced avoidance response by many phototrophic microbes (Diehn, 1973; Spudich and Bogomolni, 1984; Sprenger et al ., 1993; Lucia et al ., 1996; Armitage and Hellingwerf, 2003; Sobierajska et al ., 2006).…”

Photoresponses of Halobacterium salinarum to Repetitive Pulse Stimuli

“…As for light signals, considerable experimental evidence, obtained by a variety of experimental approaches, suggests that the basic mechanism for the activation of the receptor is the shuttling between two protein conformations (Yao et al, 1994;Rath et al, 1994;Bogomolni et al, 1994;Spudich and Lanyi, 1996;Jung and Spudich, 1996;Perazzona et al, 1996;Spudich et al, 1997;Zhang et al, 1997). However, specific interactions between signals coming from different photoreceptors or photoreceptor states have already been observed experimentally (Lucia et al, 1996(Lucia et al, , 1997. In this paper we present further results along this line, and discuss them in terms of the aforementioned models.…”

“…This scheme was also adopted to account for blue transduction (switching on a blue light pushes the system in the switching-inducing state), assuming that thermal decay of S 373 is the slowest step (Marwan et al, 1995). However, this could only occur in the orange-adapted state and could not account for the specific interactions between orange and blue stimuli, observed by using a conditioning stimulus and one or two pulse test stimuli in sequence (Lucia et al, 1996(Lucia et al, , 1997). As will be discussed further on, these results cannot be explained simply by photocycling, and other problems arise from the existence of phototaxis mutants exibiting inverted responses (Olson et al, 1995).…”

Competition-Integration of Blue and Orange Stimuli in Halobacterium salinarum Cannot Occur Solely in SRI Photoreceptor

“…The existence of two cytoplasmic signals could also account for the color specificity of reported delayed effects of stimuli, occurring after the delivery of a stimulus on a time scale much longer than that of the photocycle (Mac-Cain et al, 1987;Lucia et al, 1996Lucia et al, , 1997Cercignani et al, 1998).…”

“…Several (usually 10) files are stored to increase the number of observed cells; cumulative data are displayed as a histogram of the reversal frequency vs. time. Details on the apparatus for light stimulation and for data analysis are reported elsewhere (Lucia et al, 1996). A photoresponse index (r) is evaluated as follows:…”