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Plant and Cell Physiology, 1999, Vol. 40, No. 2 213-221
© 1999

Control by Ammonium Ion of the Change from Step-Up to Step-Down Photophobically Responding Cells in the Flagellate Alga Euglena gracilis

Shigeru Matsunaga1, Tetsuo Takahashi2, Masakatsu Watanabe3, Michizo Sugai4 and Terumitsu Hori1

1 Institute of Biological Sciences, University of Tsukuba Tsukuba, Ibaraki, 305-8572 Japan
2 Japan Advanced Institute of Science and Technology Tatsunokuchi, Ishikawa, 923-1292 Japan
3 National Institute for Basic Biology Okazaki, Aichi, 444-8585 Japan
4 Department of Biology, Faculty of Science, Toyama University Toyama, Toyama, 930-8555 Japan

The regulation between step-down and step-up photophobic responses, resulting in photoaccumulation of the cells in an actinic light trap or cells' avoidance from an excessive illumination, is crucially important for the survival of phototrophic organisms such as Euglena gracilis. As for the factors involved in this regulation in Euglena gracilis, we for the first time report here that ammonium ion specifically enhances step-down photophobic response, together with the effects of L-methionine-DL-sulfoximine (L-MSO), an inhibitor of ammonium assimilation, to specifically enhance step-up photophobic response. The apparent positive correlation between the degree of greening and the step-down photophobic response did not seem to reflect real causal relationship in view of the results with effects of gabaculine, an inhibitor of {gamma}-aminolaevulinic acid ({gamma}-ALA) formation. The transmission of stigma and step-down appearance did not show any correlation either, in contrast to a previous assumption by other authors. Cycloheximide (CHX), an inhibitor of eukaryotic protein synthesis, suppressed step-down appearance and enhanced step-up appearance, probably suggesting an involvement of some (newly synthesized) protein(s) specifically in the step-down photosignal detection and/or signal transduction process(es).

(Received August 18, 1998; Accepted December 3, 1998)
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