Abstract Regulation of the Calvin–Benson cycle under varying light/dark conditions is a common property of oxygenic photosynthetic organisms and photosynthetic glyceraldehyde-3-phosphate dehydro- genase (GAPDH) is one of the targets of this complex regulatory system. In cyanobacteria and most algae, photosynthetic GAPDH is a homotetramer of GapA subunits which do not contain regulatory domains. In these organisms, dark-inhibition of the Calvin–Benson cycle involves the formation of a kinetically inhibited supramolecular complex between GAPDH, the regu- latory peptide CP12 and phosphoribulokinase. Condi- tions prevailing in the dark, i.e. oxidation of thioredoxins and low NADP(H)/NAD(H) ratio pro- mote aggregation. Although this regulatory system has been inherited in higher plants, these phototrophs contain in addition a second type of GAPDH subunits (GapB) resulting from the fusion of GapA with the C-terminal half of CP12. Heterotetrameric A 2 B 2 - GAPDH constitutes the major photosynthetic GAP- DH isoform of higher plants chloroplasts and coexists with CP12 and A 4 -GAPDH. GapB subunits of A 2 B 2 - GAPDH have inherited from CP12 a regulatory domain (CTE for C-terminal extension) which makes the enzyme sensitive to thioredoxins and pyridine nucleotides, resembling the GAPDH/CP12/PRK sys- tem. The two systems are similar in other respects: oxidizing conditions and low NADP(H)/NAD(H) ratios promote aggregation of A 2 B 2 -GAPDH into strongly inactivated A 8 B 8 -GAPDH hexadecamers, and both CP12 and CTE specifically affect the NADPH- dependent activity of GAPDH. The alternative, lower activity with NADH is always unaffected. Based on the crystal structure of spinach A 4 -GAPDH and the anal- ysis of site-specific mutants, a model of the autono- mous (CP12-independent) regulatory mechanism of A 2 B 2 -GAPDH is proposed. Both CP12 and CTE seem to regulate different photosynthetic GAPDH isoforms according to a common and ancient molecular mechanism. Keywords Calvin-Benson cycle Æ Disulfide Æ Light/ dark regulation Æ Pyridine nucleotides Æ Supramolecular complex Æ Thioredoxin Abbreviations BPGA 1,3-bisphosphoglycerate CTE C-terminal extension of subunits GapB GapA and GapB subunits A and B, respectively, of photosynthetic glyceraldehyde-3- phosphate dehydrogenase GAPDH glyceraldehyde-3-phosphate dehydrogenase NADP-MDH NADP-dependent malate dehydrogenase PRK phosphoribulokinase P. Trost (&) Æ L. Marri Æ M. Zaffagnini Æ S. Scagliarini Æ P. Pupillo Æ F. Sparla Laboratory of Molecular Plant Physiology, Department of Evolutionary Experimental Biology, University of Bologna, Via Irnerio 42, Bologna 40126, Italy e-mail: trost@alma.unibo.it S. Fermani Æ G. Falini Department of Chemistry ‘‘G. Ciamician’’, University of Bologna, Bologna, Italy Photosynth Res (2006) 89:263–275 DOI 10.1007/s11120-006-9099-z 123 REVIEW Thioredoxin-dependent regulation of photosynthetic glyceraldehyde-3-phosphate dehydrogenase: autonomous vs. CP12-dependent mechanisms P. Trost Æ S. Fermani Æ L. Marri Æ M. Zaffagnini Æ G. Falini Æ S. Scagliarini Æ P. Pupillo Æ F. Sparla Received: 8 June 2006 / Accepted: 21 August 2006 / Published online: 22 September 2006 Ó Springer Science+Business Media B.V. 2006