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Title: LIGHT MODULATION OF RUBISCO IN ARABIDOPSIS REQUIRES REDOX REGULATION OF THE LARGER RUBISCO ACTIVASE ISOFORM

Author
item PORTIS JR, ARCHIE
item ZHANG, NING - CROP SCIENCES UOFI URBANA
item EWY, ROBERT - 3611-30-00
item KALLIS, RUSSELL - CROP SCIENCES UOFI URBANA

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2002
Publication Date: 12/20/2002
Citation: PORTIS JR, A.R., ZHANG, N., EWY, R.G., KALLIS, R.P. LIGHT MODULATION OF RUBISCO IN ARABIDOPSIS REQUIRES REDOX REGULATION OF THE LARGER RUBISCO ACTIVASE ISOFORM. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. 2002. v. 99. p. 330-3334.

Interpretive Summary: The activity of Rubisco, the enzyme that captures carbon dioxide, often limits photosynthesis, the process by which plant use light energy from the sun to make carbohydrates for growth from carbon dioxide and water. In order to have activity, Rubisco requires modification by a process called activation, which is regulated by another protein known as Rubisco activase. Most plants contain two isoforms of the activase that are created by alternative splicing during gene transcription. In this study, we found that the regulation of Rubisco is deregulated in transgenic plants modified to express only one of the activase isoforms, such that Rubisco always has its maximal activity in the light. Further studies of these plants should indicate if plant growth could be increased by this simple but fundamental alteration in the regulation of Rubisco. This information will benefit scientists attempting to modify the properties and regulation of Rubisco in ways beneficial for increased photosynthesis by crop plants.

Technical Abstract: The light activation of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in vivo requires the presence of Rubisco activase, a nuclear-encoded chloroplast protein that consists of two isoforms arising from alternative splicing in most plants. We examined the function of each isoform by characterizing Rubisco activation in transgenic Arabidopsis plants that express only one or both isoforms, as compared to the wildtype. In plants expressing only the shorter isoform, Rubisco activity was as high as in the wildtype under saturating light, but the activity was not down-regulated at intensities limiting for photosynthesis. In contrast in plants expressing only the longer isoform, Rubisco activity was down-regulated at limiting light, but the activity was slightly lower and increased much more slowly at saturating light intensities a compared to the wildtype. When the capacity to redox regulate the activity of the larger activase isoform was eliminated by replacement of the critical cysteine residues in the carboxy-terminal extension unique to this isoform, Rubisco activity in saturating light was similar to the wildtype, bu the ability of the larger isoform to down-regulate Rubisco activity at limiting light intensities in transgenic plants was almost abolished. These results indicate that the light modulation of Rubisco under limiting light is mainly due to the ability to regulate the activity of Rubisco activase by redox changes in the stroma.