ИСТИНА

Analysis of behavioral and electric responses shows that phototaxis in cryptophytes is likely mediated by a two-rhodopsin-based photosensory mechanism similar to that recently demonstrated in the green alga Chlamydomonas reinhardtii. The marine cryptophyte Guillardia theta exhibits positive phototaxis with maximum sensitivity at 450 nm and a secondary band above 500 urn. Variability of the relative sensitivities at these wavelengths and light-dependent inhibition of phototaxis in both bands by hydroxylamine suggest the involvement of two rhodopsin photoreceptors. Two photoreceptor currents similar to those mediated by sensory rhodopsins in green algae were recorded in the related freshwater cryptophyte Cryptomonas sp. S2. Two cDNA sequences from G. theta and one from Cryptomonas sp. S2 encoding proteins homologous to type I opsins were identified. The presence of introns in the respective genomic copies rules out their possible origin from prokaryotic contamination. One of the G. theta sequences, designated GtRI, was expressed in photoactive form in Escherichia coli, which for the first time, to the best of our knowledge, made possible optical spectroscopy and charge movement measurements in an algal rhodopsin. The photochemical reaction cycle of the E. coli-expressed GtRI involves K-, M-, and O-like intermediates with relatively slow (-80 ms) turnover time. GtRI shows lack of light-driven proton pumping activity in E. coli cells, although carboxylate residues are at the positions of the Schiff base proton acceptor and donor as in proton-pumping rhodopsins. This result together with the absorption spectrum that corresponds to the longwavelength band of the phototaxis spectral sensitivity makes this pigment a candidate for one of G. theta phototaxis receptors. A second rhodopsin from G. theta (GtR2) and the one from Cryptomonas sp. S2 have non-carboxylate residues atthe donor position as in known sensory rhodopsins, whicl is in line with their possible function as phototaxis receptors.