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PLANT PHYSIOLOGY , Vol 112, Issue 2 677-684, Copyright © 1996 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
A Low-CO2-Inducible Gene Encoding an Alanine:[alpha]-Ketoglutarate Aminotransferase in Chlamydomonas reinhardtii
Z. Y. Chen, M. D. Burow, C. B. Mason and J. V. Moroney
Department of Plant Biology, Louisiana State University, Baton Rouge, Louisiana 70803
At low-CO2 (air) conditions, the unicellular green alga Chlamydomonas
reinhardtii acquires the ability to raise its internal inorganic carbon
concentration. To study this adaptation to low CO2, cDNA clones induced
under low-CO2 growth conditions were selected through differential
screening. One full-length clone is 2552 bp, with an open reading frame
encoding 521 amino acids. The deduced amino acid sequence shows about 50%
identity with alanine:[alpha]-ketoglutarate aminotransferase (Ala AT, EC
2.6.1.2) from plants and animals, and the mRNA of this clone increased 4-
to 5-fold 4 h after cells were switched from high-CO2 to low-CO2 growth
conditions. The expression of the enzyme and its activity also increased
accordingly at low-CO2 growth conditions. To study the physiological role
of Ala AT, a pyridoxal phosphate inhibitor, aminooxyacetic acid, was added
at 40 [mu]M to the growth medium when cells were beginning to adapt to low
CO2. This caused a 30% decrease in the maximum photosynthetic rate in
air-adapting cells 8 h later. The addition of the inhibitor also caused the
cells to excrete glycolate, a photorespiratory intermediate, but did not
change the apparent affinity of the cell for external CO2. These
physiological studies are consistent with the assumption that Ala AT is
involved in the adaptation to low-CO2 conditions.
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