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PLANT PHYSIOLOGY , Vol 109, Issue 2 681-685, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Pseudoreversion Substitution at Large-Subunit Residue 54 Influences the CO2/O2 Specificity of Chloroplast Ribulose-Bisphosphate Carboxylase/Oxygenase
R. J. Spreitzer, G. Thow and G. Zhu
Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588-0664
Chlamydomonas reinhardtii mutant 31-4E lacks ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco;EC 4.1.1.39) holoenzyme due to a mutation in
the chloroplast rbcL gene. This mutation causes a glycine54-to-aspartate
substitution within the N-terminal domain of the Rubisco large subunit. In
the present study, photosynthesis-competent revertants were selected to
determine whether other amino acid substitutions might complement the
primary defect. Revertants were found to arise from only true reversion or
either of two forms of pseudoreversion affecting residue 54. One
pseudorevertant has a glycine54-to-alanine substitution that decreases the
accumulation of holoenzyme, but the purified Rubisco has near-normal
kinetic properties. The other pseudorevertant has a glycine54-to-valine
substitution that causes an even greater decrease in holoenzyme
accumulation. Rubisco purified from this strain was found to have an 83%
decrease in the Vmax of carboxylation and an 18% decrease in the CO2/O2
specificity factor. These results indicate that small increases in the size
of amino acid side chains can influence Rubisco assembly or stability. Even
though such changes occur far from the active site, they also play a
significant role in determining Rubisco catalytic efficiency.
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