Following centrifugation for 20 min at 26,000×g, protein in the extract was precipitated with 80 % ammonium sulphate, collected by centrifugation and suspended in buffer A. Following desalting on a Sephadex G-25 column, the dPGM-ST was purified by passage over a 20 ml column of Strep-tactin Sepharose (IBA GmbH, Goettingen, Germany) that had been equilibrated in buffer A. After washing with 10 column volumes of buffer A, dPGM-ST was eluted with 5 mM desthiobiotin in buffer A. The purified dPGM-ST was precipitated with ammonium sulphate and desalted on a Sephadex G-25 column, equilibrated

with 60 mM Tris–HCl, pH 7.9. Fractions containing protein were pooled and stored at −80 °C. For the initial development of the assay, PEP carboxylase was purified from maize leaves by a procedure described for Rubisco (Carmo-Silva et al. 2011). The protein peak corresponding to PEP carboxylase eluted from the ion-exchange column BGB324 research buy just prior to that of Rubisco. A commercially available PEP carboxylase (Sigma–Aldrich #C1744) from a microbial source was also used in the assay. Measurement of RCA activity using purified proteins RCA activity was measured as the ability to restore

activity to the inactive PF-562271 nmr Rubisco-RuBP (ER) complex (Salvucci et al. 1985). Rubisco activity was measured in reactions containing 100 mM Tricine-NaOH, pH 8, 10 mM MgCl2, 10 mM NaHCO3, 5 mM DTT, 5 % (w/v) PEG-3350, 1 mM NADH, 0.48 U enolase, 0.75 U dPGM-ST, 0.2 mM 2,3-bisPGA, 2 mM RuBP, 10 mM glucose-6-phosphate, 0.75 U PEP carboxylase, 1 U malic dehydrogenase, 5 mM ATP plus ADP at various ratios, and recombinant RCA and Rubisco at the concentrations indicated in the text.

For assays using the commercially available microbial PEP carboxylase, the microbial PEP carboxylase (1 U) was substituted for the maize enzyme and glucose-6-phosphate and PEG-3350 were Dichloromethane dehalogenase omitted from the mix. To avoid under-estimating activity and to eliminate long lags in product conversion, the specific activities of the linking enzymes were more than tenfold higher than the maximum activity of Rubisco at the highest concentration used. When tested using sub-saturating and saturating concentrations of 3-PGA, the activities of the linking enzymes catalysed NADH oxidation at rates that were several-fold higher than the maximum rate of Rubisco activity. Rubisco assays were conducted at 30 °C in 96 well plates in a total volume of 0.1 or 0.2 mL. RCA was added to reactions containing all of the components except Rubisco. After 30 s, reactions were initiated with Rubisco in the ER form and the decrease in absorbance at 340 nm, linked to the stoichiometric production of 3-PGA, was measured continuously using a Synergy HT (Bio-Tek, Denkendorf, Germany) plate reader. To determine the activity of the fully carbamylated ECM form, reactions were first incubated for 3 min without RuBP.