title: Rubisco mutagenesis provides new insight into limitations on photosynthesis and growth in Synechocystis PCC6803.
abstract: Orthophosphate (Pi) stimulates the activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) while paradoxically inhibiting its catalysis. Of three Pi-binding sites, the roles of the 5P- and latch sites have been documented, whereas that of the 1P-site remained unclear. Conserved residues at the 1P-site of Rubisco from the cyanobacterium Synechocystis PCC6803 were substituted and the kinetic properties of the enzyme derivatives and effects on cell photosynthesis and growth were examined. While Pi-stimulated Rubisco activation diminished for enzyme mutants T65A/S and G404A, inhibition of catalysis by Pi remained unchanged. Together with previous studies, the results suggest that all three Pi-binding sites are involved in stimulation of Rubisco activation, whereas only the 5P-site is involved in inhibition of catalysis. While all the mutations reduced the catalytic turnover of Rubisco (K(cat)) between 6- and 20-fold, the photosynthesis and growth rates under saturating irradiance and inorganic carbon (Ci) concentrations were only reduced 40-50% (in the T65A/S mutants) or not at all (G404A mutant). Analysis of the mutant cells revealed a 3-fold increase in Rubisco content that partially compensated for the reduced K(cat) so that the carboxylation rate per chlorophyll was one-third of that in the wild type. Correlation between the kinetic properties of Rubisco and the photosynthetic rate (P(max)) under saturating irradiance and Ci concentrations indicate that a >60% reduction in K(cat) can be tolerated before P(max) in Synechocystsis PCC6803 is affected. These results indicate that the limitation of Rubisco activity on the rate of photosynthesis in Synechocystis is low. Determination of Calvin cycle metabolites revealed that unlike in higher plants, cyanobacterial photosynthesis is constrained by phosphoglycerate reduction probably due to limitation of ATP or NADPH.
authors.affiliation: Department of Molecular Biology and Ecology of Plants, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel. yehoudam@post.tau.ac.il
authors.forename: Yehouda
authors.initials: Y
authors.lastname: Marcus
authors.forename: Hagit
authors.initials: H
authors.lastname: Altman-Gueta
authors.forename: Yael
authors.initials: Y
authors.lastname: Wolff
authors.forename: Michael
authors.initials: M
authors.lastname: Gurevitz
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D001665:Binding Sites; D055162:Biocatalysis / Q000187:drug effects; D002734:Chlorophyll / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D007700:Kinetics; D016296:Mutagenesis / Q000187:drug effects; D050505:Mutant Proteins / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D010710:Phosphates / Q000494:pharmacology; D010788:Photosynthesis / Q000187:drug effects / Q000235:genetics*; D011485:Protein Binding / Q000187:drug effects; D012273:Ribulose-Bisphosphate Carboxylase / Q000235:genetics* / Q000378:metabolism; D046939:Synechocystis / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development* / Q000648:ultrastructure
chemical_list: D050505:Mutant Proteins; D010710:Phosphates; D002734:Chlorophyll; D012273:Ribulose-Bisphosphate Carboxylase; D010100:Oxygen