In Vivo Absorption Spectra of the Two Stable States of the Euglena Photoreceptor Photocycle

Abstract

Euglena gracilis possesses a simple but sophisticated light detecting system, consisting of an eyespot formed by carotenoids globules and a photoreceptor. The photoreceptor of Euglena is characterized by optical bistability, with two stable states. In order to provide important and discriminating information on the series of structural changes that Euglena photoreceptive protein(s) undergoes inside the photoreceptor in response to light, we measured the in vivo absorption spectra of the two stable states A and B of photoreceptor photocycle. Data were collected using two different devices, i.e. a microspectrophotometer and a digital microscope. Our results show that the photocycle and the absorption spectra of the photoreceptor possess strong spectroscopic similarities with a rhodopsin-like protein. Moreover, the analysis of the absorption spectra of the two stable states of the photoreceptor and the absorption spectrum of the eyespot suggests an intriguing hypothesis for the orientation of microalgae toward light.

Unlocking the biological potential ofEuglena gracilis: evolution, cell biology and significance to parasitism

“…Most Euglena-specific predicted membrane proteins are single-copy (87.5%), while there are clear large families that possess conserved features, as discussed above. Notably, we were unable to identify a rhodopsin homolog, which is in contrast to several biochemical analyses that have suggested the presence of retinal, the rhodopsin cofactor, and which have been interpreted as evidence for a rhodopsin-like light sensing transduction mechanism (Barsanti et al, 2009).…”

Algae through the looking glass

“…The spectra of the states A and B, measured by microspectrophotometry and digital microscopy, were superimposable. The state A shows a band centered at 498 nm, the state B shows a band centered at 462 nm with a blue shift of 36 nm (Barsanti et al, ). Figure a shows the image of the photoreceptor of Euglena (framed region) recognized as an almond‐shaped dark‐gray organelle inside the apical part of the cell.…”

Fundamental questions and concepts about photoreception and the case of Euglena gracilis

“…The wavelengths in the positive part of the spectrum drive the transition of A 498 to B 462 , while the wavelengths in the negative part drive the opposite transition. 64 Another positive region exists in the UV, below 400 nm, as reported by Barsanti et al (1997), 63 which these authors couldn't measure by microspectroscopy, because of the extremely complex instrumental setup necessary for this technique.…”

“…The presence of optical bistability together with a fluorescent form is a common feature of euglenoid photoreceptors, present also in Phacus and Trachelomonas. 64 Though the extraction of photoreceptor protein has not yet been reported, the extraction of retinal from Euglena photoreceptor sample, 65 together with the matching between the spectral properties and structural peculiarities of Euglena photoreceptor protein and those of proteorhodopsin, 61,66 allowed the authors to confirm that this organelle is a threedimensional assemblage of one photoreceptive protein belonging to the superfamily of rhodopsin-like proteins.…”

“…These spectroscopic characteristics were assessed also by digital microscopy, 64 which provided a direct visualization of the different absorbance and the shift between the spectra of the two forms of the photoreceptor proteins.…”