Recent Publications

• Zhang, N., Kallis, R.P., Ewy, R.G. and Portis, A.R. Jr. (2002) Light modulation of Rubisco in Arabidopsis requires redox regulation of the larger Rubisco activase isoform. Proc. Natl. Acad. Sci. USA 99:3330-3334
• Bernacchi, C.J., Singsaas, E.L., Pimentel, C., Portis, A.R. Jr., and Long, S.P. 2001 Improved temperature response functions for models of Rubisco-limited photosynthesis. Plant Cell Environ. 24: 253-259
• Zhang, X.-H., Widholm, J.M., and Portis, A.R. Jr. 2001 Photosynthetic properties of two different soybean suspension cultures. J. Plant Physiol. 158: 357-365
• Zhang, N., Schürmann, P., and Portis, A.R. Jr. 2001 Characterization of the regulatory function of the 46-kDa isoform of Rubisco activase from Arabidopsis. Photosynth. Res. 68: 29-37
• Zhang, X.-H., Brotherton, J.E., Widholm, J.M., and Portis, A.R. Jr. 2001 Targeting a nuclear anthranilate synthase a-subunit gene (ASA2) to the tobacco plastid genome results in enhanced tryptophan biosynthesis - return of a gene to its pre-endosymbiotoc origin. Plant Physiol., 127:131-141
• Zhang, X-.H., Portis, A.R. Jr., and Wildholm, J.M. 2001 Plastid transformation of soybean suspension cultures. J. Plant Biotechnology 3: 39-44
• Portis, A.R. Jr. 2001 The Rubisco activase - Rubisco system: an ATPase dependent association that regulates photosynthesis. In: Protein - Protein Interactions in Plant Biology, M. McManus, W. Laing and A. Allen eds., Annual Plant Rev. Vol 7, Sheffield Academic Press, Sheffield England
• Kallis, R.P., Ewy, R.G., & Portis, A.R. Jr. 2000. Alteration of the adenine nucleotide response and increased Rubisco activation activity of Arabidopsis Rubisco activase by site-directed mutagenesis. Plant Physiol. 123: 1077-1086
• Ott, C.M., Smith, B.D., Portis, A.R. Jr. & Spreitzer, R.J. 2000. Activase region on chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase. Nonconservative substitution in the large subunit alters species specificity of protein interaction. J. Biol. Chem. 275: 26241-26244
• Zhang, N. & Portis, A.R. Jr. 1999. Mechanism of light activation of Rubisco: A specific role for the larger Rubisco activase isoform involving reductive activation by thioredoxin-f. Proc. Natl. Acad. Sci. USA 96: 9438-9443
• Kane, H.J., Wilkin, J.-M., Portis, A.R. Jr. & Andrews,T.J. 1998. Potent inhibition of ribulose-bisphosphate carboxylase by an oxidized impurity in ribulose-1,5-bisphosphate. Plant Physiol. 117: 1059-1069
• Esau. B.D., Snyder, G.W. & Portis, A.R. Jr. 1998. Activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) with chimeric activase proteins. Photosynth. Res. 58:175-181
• Larson, E.M., O'Brien, C.M., Zhu, G., Spreitzer, R.J., & Portis, A.R. Jr. 1997. Specificity for activase is changed by a Pro-89 to Arg substitution in the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. J. Biol. Chem. 272:17033-17037
• Eckardt, N.A., Snyder, G.W., Portis, A.R. Jr., & Ogren W.L. 1997. Growth and photosynthesis under high and low irradiance of Arabidopsis thaliana antisense mutants with reduced ribulose-1,5-bisphosphate carboxylase/oxygenase activase content. Plant Physiol. 113:575-586
• Lilley, R.M. and Portis, A.R. Jr. 1997. ATP hydrolysis activity and polymerization state of ribulose-1,5-bisphosphate carboxylase oxygenase activase. Do the effects of Mg2+, K+ and activase concentrations indicate a functional similarity to actin? Plant Physiol. 114:605-613
• Esau, B.D., Snyder, G.W., and Portis, A.R. Jr. 1996. Differential effects of N- and C-terminal deletions on the two activities of Rubisco activase. Arch. Biochem. Biophys. 326:100-105
• Portis, A.R. Jr. 1995. The regulation of Rubisco by Rubisco activase. J. Exp. Bot. 46:1285-1291

Research Interests

Photosynthetic carbon metabolism is is initiated by the enzyme, Rubisco, that combines atmospheric carbon dioxide with a sugar phosphate, ribulose bisphosphate, in a reaction called carboxylation. The photosynthetic potential and efficiency of plants can be limited by the activity of Rubisco over a wide variety of conditions. For example, the carboxylation reaction appears to be difficult because the maximal rate of the enzyme is very slow compared to most other metabolic enzymes. To compensate for this, plants invest about 25% of the nitrogen present in their leaves in the synthesis of this single protein. Also, Rubisco is not able to prevent oxygen from reacting with the ribulose bisphosphate during the reaction, allowing oxgenation to occur instead of carboxylation which further reduces photosynthetic potential. Plants recycle the product of this reaction in a process called photorespiration, but nevertheless must release one carbon dioxide molecule for every two oxygen molecules that react. The oxygenase reaction also reduces photosynthetic potential under limiting light conditions because photorespiration, like photosynthesis, requires energy. Finally, it is often advantageous to reduce (i.e. regulate) the activity of Rubisco when environmental conditions are not favorable (e.g. light is limiting or the products cannot be used adequately). This regulation is achieved by altering the activity of another protein, called Rubisco activase. Usually regulation comes at some expense to achieving maximal performance. Whether this is the case with Rubisco regulation is not yet clear because several aspects of the regulatory process are not adequately understood.

Therefore research in my laboratory is directed at the goal of improving the photosynthetic potential and efficiency of plants by altering the properties and regulation of Rubisco. In order to effect such changes, a fairly detailed knowledge of the properties and regulation is required in order to know what changes to make and a means to introduce these changes into the plant is required. Currently, we have projects that are directed at both these problems and they are summarized below:

1. We are developing methods to replace the Rubisco protein present in soybean with another. Genetic engineering of Rubisco is difficult at the whole plant level because it involves chloroplast transformation and this has only recently been developed in tobacco. We are attempting to extend these methods to soybean and replace the natural enzyme with one predicted to be more suitable for the higher carbon dioxide levels that will exist in the near future. This approach exploits our current knowledge that natural variation exists in both maximal activity and relative ratio of carboxylation to oxygenation even though the exact structural changes responsible for these kinetic differences are not understood.
2. We are determining the effects of altered regulation of Rubisco on photosynthetic potential. Studies of the structure, activity and regulation of Rubisco activase have resulted in the creation of a protein that is not regulated in one manner like the normal protein yet retains high activity. Arabidopsis plants which are transformed with this mutant protein have been obtained and are now being characterized to learn more about the role of Rubisco regulation in determining photosynthesis and growth.
3. We are continuing to characterize and thus understand the regulation of Rubisco activase activity and how this protein regulates the activity of Rubisco. Major areas of current investigation are the role of thylakoids and light in regulation of Rubisco activase, studies of the interaction between Rubisco and Rubisco activase using site-directed mutagenesis, and a more detailed investigation of the effects of moderate heat stress on plants that appears to inhibit photosynthesis by reducing the activation of Rubisco.

Return to previous page.