AN EXTREME-XANTHORHODOPSIN FROM SALINIVIBRIO SOCOMPENSIS, A HALOPHILIC STRAIN ISOLATED FROM MODERN STROMATOLITES

GORRITI M.; BAMANN, C.; ALONSO REYES D.; WOOD, P.G.; BAMBERG, E; FARIAS M.E.; GARTNER, WOLFGANG; ALBARRACÍN V. H.

Photochemical and Photobiological Sciences

ROYAL SOC CHEMISTRY

Lugar: CAMBRIDGE; Año: 2023 p. 1 - 15

1474-905X

A putative xanthorhodopsin-encoding gene, XR34, was found in the genome of the moderately halophilic gammaproteobacterium Salinivibrio socompensis S34, isolated from modern stromatolites found on the shore of Laguna Socompa (3,570 m), Argentina Puna. XR-encoding genes were clustered together with genes encoding Xcarotene, retinal (vitamin-A aldehyde), and carotenoid biosynthesis enzymes while the carotene ketolase gene critical for the salinixanthin antenna compound was absent. To identify its functional behaviour, we herein overexpressed and characterized this intriguing microbial rhodopsin. Recombinant XR34 showed all the salient features of canonical microbial rhodopsin and covalently bound retinal as a functional chromophore with max = 561 nm (max ca. 60´000 M-1 cm-1). Two canonical counterions with pK values of around 6 and 3 were identified by pH titration on the recombinant protein. With a recovery time of approximately half an hour in the dark, XR34 shows light-dark adaptation shifting the absorption maximum from 551 nm to 561 nm. Laser-flash induced photochemistry at pH 9 (deprotonated primary-counterion) identified a photocycle starting with a K-like intermediate, followed by a M state(λmax ca. 400 nm, deprotonated Schiff base), and a final long wavelength absorbing N- or O-like intermediate before returning to the parental 561 nm state. Initiating the photocycle at pH 5 (protonated counterion) yields only bathochromic intermediates, due to the lacking capacity of the counterion to accept the Schiff base proton. Illumination of the membrane-embedded protein yielded a capacitive transport current. The presence of the M-intermediate under these conditions was demonstrated by a blue light-induced shunt process.