Photosensory transduction in the flagellated alga, Euglena gracilis. II. Evidence that blue light effects alteration in Na+/K+ permeability of the photoreceptor membrane.

1. The blue light-induced cell tumbling behavior (the step-down photophobic response) and the accumulation of cells into a blue light trap (photoaccumulation) were investigated in Euglena. Dose response plots for these phenomena which we collectively term 'photobehavior' show both threshold and saturation characteristics. 2. NaCl effects apparent elevation in the photosensitivity of the cell as evidenced by alteration of the dose response plot character and lowering of the light intensity saturation level. 3. NaCl and ouabain enhance the duration of the photophobic responses and the rate of photoaccumulation. KCl and NH4Cl have lesser or inhibitory effects. 4. Choline chloride reduces the duration of the photophobic responses and the rate of photoaccumulation. 5. KCl reduces the enhancement of photobehavior induced by NaCl and at constant chloride concentration, photobehavior is unaffected by the relative KCl and NaCl concentrations. 6. Antagonists of voltage-dependent, monovalent cation fluxes in membranes (tetrodotoxin, procaine, tetraethylammonium, 4-aminopyridine) do not alter photobehavior. 7. The results suggest a role for a photoreceptor membrane-located transport system for Na+/K+ as a key step in control of the intraflagellar free Ca/+ levels that determine the photobehavior mediated by flagellar reorientation.

Photosensory transduction in the flagellated alga, Euglena gracilis I. Action of divalent cations, Ca2+ antagonists and Ca2+ ionophore on motility and photobehavior.

1. The flagellated alga, Euglena gracilis, swims forward essentially in a straight path under constant light intensity. Strong motility of the cells can be supported by Mg2+ alone but optimum motility is found in the presence of Mg2+, Ca2+ and K+. 2. Ca2+, Co2+, Mn2+ and Ba2+ induce a concentration-dependent increase in the rate at which the cells change the direction of their swimming path (a klinokinesis). Ni2+ immobilizes the flagellum. 3. On perception of a reduction ('step-down stimulus') in blue light intensity in their environment, Euglena rotate in place (tumble) for a finite period (the step-down photophobic response). 4. The duration of the tumbling is enhanced in the presence of divalent cations following the series Ca2+ greater than Ba2+ greater than Mn2+ greater than Co2+ greater than Mg2+ = Ni2+ = 0. 5. Neither the tumbling response in the presence of low concentrations of Ca2+ or the Ca2+-stimulated response is altered by verapamil (a Ca2+ conductance antagonist). The Ca2+ conductance/active transport antagonist, ruthenium red, is also inactive. 6. The Ca2+ ionophore, A23187, has little effect on flagellar activity in the absence of extracellular Ca2+. However, in the presence of A23187, Ca2+ induces a specific light-independent, concentration-dependent discontinuous tumbling response of the cells. 7. The data support a role for Ca2+ and Mg2+ in control of flagellar activity. However, blue light-induced tumbling behavior would not appear to be the direct result of a light-mediated alteration in the Ca2+ conductance of the flagellar membrane to affect flagellar reorientation. The results are discussed in connection with previous theories on control of flagella activity in green alga.

Cytochrome c oxidase as the receptor molecule for chemoaccumulation (chemotaxis) of Euglena toward oxygen.

Chemoaccumulation of Euglena gracilis toward oxygen was selectively inhibited, without concomitant effects on cell motility, by cyanide (10(-6) to 10(-4) molar) and carbon monoxide (5 x 10(-5) to 5 x 10(-4) molar). Above these concentrations, motility of the cell was imparied and the chemosensory response was inhibited. Azide did not affect chemoaccumulation even at 5 x 10(-3) molar. It is concluded that cytochrome a3 serves as the chemoreceptor molecule for oxygen-mediated behavioral responses in Euglena.

Phototaxis and sensory transduction in Euglena.

The accumulation of Euglena gracilis in an illuminated region is brought about by two main mechanisms: orientation and subsequent directed movement (positive phototaxis) toward light scattered from particles in the illuminated zone; and by the trapping of cells in this region because of shock reactions experienced upon the cells encountering a sudden decrease of light intensity at the light-dark boundary (inverse photophobic responses). Phototactic orientation is mediated by inverse photophobic reactions which occur when the shadow of the stigma periodically falls upon the photoreceptor proper. Euglena also exhibits shock reactions when an already high light intensity is increased further (direct photophobic responses). The expression of both types of phobic responses depends upon stimulus intensity and adaptation of the sensory system in a seemingly complex way. A definition of the minimum components of the stimulus transduction system and a systems analytical approach to the study of input-output relationships enables one to construct an electronic analog of the cell's signal processing system that converts the photoreceptor input to commands which activate or inhibit flagellar reorientation. Computer simulation studies show that this model has considerable predictive value. It is hoped that with the approach presented in this article, a generalized model has become available for dealing with the questions of sensory transduction in aneural systems. Certainly, at this point more questions have been raised than have been answered. Where is the processing device located? Are its kinetic properties determined by electrical processes or by the rates of chemical reactions? Is the processor, and thereby the behavior of the orgamism, modulated by natural environmental parameters, and can it be modified permanently through more drastic chemical treatment of the cell? Is the system capable of permanent or transitory modification through repeated response, that is, does it exhibit phenomena analogous to learning and memory in higher organisms? These are only a few of the problems that require study in the future.