The ability of photosynthetic organisms to use CO 2 for photosynthesis depends in part on the properties of Rubisco. Rubisco has a surprisingly poor affinity for CO 2 , probably because it evolved in an atmosphere that had very high CO 2 levels compared with the present atmosphere. In C 3 plants the K m (CO 2 ) of Rubisco ranges between 15 and 25 m. In cyanobacteria Rubisco has an even lower affinity for CO 2 , and the K m (CO 2 ) can be greater than 200 m. In comparison, the concentration of CO 2 in water in equilibrium with air is approximately 10 m. From these numbers it becomes apparent that Rubisco is operating at no more than 30% of its capacity under standard atmospheric conditions. This is one of the reasons that C 3 plants contain such large amounts of Rubisco. Exacerbating this situation is the fact that O 2 is a competitive substrate with respect to CO 2 .In the atmosphere, where the O 2 level is 21% and the CO 2 level is 0.035%, the competition by O 2 accounts for as much as 30% of the reactions catalyzed by Rubisco. A number of photosynthetic organisms have developed ways to increase the level of CO 2 at the location of Rubisco in the plant. This results in an increase in CO 2 fixation and a decrease in the deleterious oxygenation reaction. An excellent example of a CO 2 -concentrating mechanism in higher plants is C 4 photosynthesis, which has arisen independently in a number of plant families. Aquatic photosynthetic organisms such as the microalgae have also adapted to low CO 2 levels by concentrating CO 2 internally. This Update will focus on CO 2 -concentrating mechanisms in the microalgae. For more detailed reviews of the CO 2 concentration by algae, the reader is referred to the special issue of the Canadian Journal of Botany (1998, Vol. 76) and the article by Raven (1997). TYPES OF CO 2 -CONCENTRATING MECHANISMS AND THE PROBLEM OF LEAKAGE OF ACCUMULATED CO 2C 4 plants are the best-studied organisms that concentrate CO 2 to enhance the carboxylation reaction of Rubisco. They have high levels of PEP carboxylase in leaf mesophyll cells, whereas Rubisco is located primarily in the bundle-sheath cells. CA within the mesophyll converts CO 2 entering the leaf into HCO 3 Ϫ , which is the substrate for PEP carboxylase. The advantages that PEP carboxylase has over Rubisco are its high affinity for HCO 3 Ϫ and its insensitivity to O 2 . At physiological CO 2 levels and pH, the HCO 3 Ϫ concentration in the cytoplasm of mesophyll cells is about 50 m, whereas the K m (HCO 3 Ϫ ) of PEP carboxylase is estimated to be about 8 m. Therefore, in contrast to Rubisco, PEP carboxylase is saturated for HCO 3 Ϫ at ambient CO 2 levels. To finish the CO 2 -concentrating effect of C 4 metabolism, the C 4 acid generated in the mesophyll cells is then transported to the bundle-sheath cells and decarboxylated, creating an elevated CO 2 level specifically within these cells.The problem faced by all photosynthetic organisms that concentrate CO 2 is that it can easily diffuse through biological membranes. How can such a slipp...

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SummaryBiosynthesis of chloroplast proteins is to a large extent regulated post-transcriptionally, and a number of nuclear-encoded genes have been identified that are required for translation or stability of specific chloroplast mRNAs. A nuclear mutant of Chlamydomonas reinhardtii, hf261, deficient in the translation of the psbA mRNA, has reduced association of the psbA mRNA with ribosomes and is deficient in binding of the chloroplast localized poly (A) binding protein (cPAB1) to the psbA mRNA. Cloning of the hf261 locus and complementation of hf261 using a wt genomic clone has identified a novel gene, Tba1, for translational affector of psbA. Strains complemented with the wt Tba1 gene restore the ability of the psbA mRNA to associate with ribosomes, and restores RNA binding activity of cPAB1 for the psbA mRNA. Analysis of the Tba1 gene identified a protein with significant homology to oxidoreductases. The effect of Tba1 expression on the RNA binding activity of cPAB1, and on the association of psbA mRNA with ribosomes, implies that Tba1 functions as a redox regulator of cPAB1 RNA binding activity to indirectly promote psbA mRNA translation initiation. A model of chloroplast translation incorporating Tba1 and other members of the psbA mRNA binding complex is presented.

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