Molecular and behavioral studies on signal transducing mechanisms of photomovement responses in microorganisms

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During the period, the following new results were established as to various blue/blue-green photoreceptor-containing photomovement systems of microorganisms.(1)The regulation system of the step-down and the step-up photophobic responses in the flagellate alga Euglena was for the first time revealed to be controlled by exogenous ammonium ion. Ammonium ion specifically increases the fraction of the step-down photophobically cells, whereas L-methionine-DL-sulfoximine, an inhibitor of ammonium assimilation, specifically increases the fraction of step-up photophobically responding cells in a cell suspension. This allowed us to measure independently the true action spectra for both step-up and step-down responses. The results suggested that the two responses are mediated by flavin-containing photoreceptors, having slightly different spectroscopic characters.(2)Despite the reported fact that the receptors for photomovement responses of unicellular green alga Chlamydomonas has little homology … More to archaebacterial rhodopsins, our analog-reconstitution study has revealed a highly similar stereochemical architecture along the retinal binding site between the chlamydomons photoreceptor and halobacterial phoborhodopsin. Though this may suggest a possibility for unidentified photomovement receptor in Chlamydomonas having high homology to phoborhodopsin, we tentatively ascribed the result to a possible convergence in the evolution of blue-green sensitive photoreceptors, This is in accord with several lines of evidence that a wide variety of photoreceptor species mediates photomovement responses of various microorganisms to blue/blue-green light.(3)PYP is one of the unique photoreceptors in eubacteria. We measured step-up photophobic response of Ectothiorhodospira halophila, containing PYP, and found that the action spectrum coincided with the absorption spectrum of PYP.(4)A putative transducer-like protein mediating photophobic response of Ectothiorhodospira halophila has been cloned.(5)A new member of phoborhodopsin was cloned from a halobacterial species from Australian salt lake.(6)Phoborhodopsin from Natronobacteriurm pharaonis has been expressed in E.coil, and the recombinant protein is satisfactory purified by histidine tagging. Less