TY  - THES
A1  - Diez Cocero, Mercedes
T1  - Analysis of Rubisco – carbonic anhydrase fusions in tobacco as an approach to reduce photorespiration
N2  - Rubisco catalyses the first step of CO2 assimilation into plant biomass. Despite its crucial role, it is notorious for its low catalytic rate and its tendency to fix O2 instead of CO2, giving rise to a toxic product that needs to be recycled in a process known as photorespiration. Since almost all our food supply relies on Rubisco, even small improvements in its specificity for CO2 could lead to an improvement of photosynthesis and ultimately, crop yield. In this work, we attempted to improve photosynthesis by decreasing photorespiration with an artificial CCM based on a fusion between Rubisco and a carbonic anhydrase (CA). 
A preliminary set of plants contained fusions between one of two CAs, bCA1 and CAH3, and the N- or C-terminus of RbcL connected by a small flexible linker of 5 amino acids. Subsequently, further fusion proteins were created between RbcL C-terminus and bCA1/CAH3 with linkers of 14, 23, 32, and 41 amino acids. The transplastomic tobacco plants carrying fusions with bCA1 were able to grow autotrophically even with the shortest linkers, albeit at a low rate, and accumulated very low levels of the fusion protein. On the other hand, plants carrying fusions with CAH3 were autotrophic only with the longer linkers. The longest linker permitted nearly wild-type like growth of the plants carrying fusions with CAH3 and increased the levels of fusion protein, but also of smaller degradation products. 
The fusion of catalytically inactive CAs to RbcL did not cause a different phenotype from the fusions with catalytically active CAs, suggesting that the selected CAs were not active in the fusion with RbcL or their activity did not have an effect on CO2 assimilation. However, fusions to RbcL did not abolish RbcL catalytic activity, as shown by the autotrophic growth, gas exchange and in vitro activity measurements. Furthermore, Rubisco carboxylation rate and specificity for CO2 was not altered in some of the fusion proteins, suggesting that despite the defect in RbcL folding or assembly caused by the fusions, the addition of 60-150 amino acids to RbcL does not affect its catalytic properties. On the contrary, most growth defects of the plants carrying RbcL-CA fusions are related to their reduced Rubisco content, likely caused by impaired RbcL folding or assembly. Finally, we found that fusions with RbcL C-terminus were better tolerated than with the N-terminus, and increasing the length of the linker relieved the growth impairment imposed by the fusion to RbcL. Together, the results of this work constitute considerable relevant findings for future Rubisco engineering.
N2  - Rubisco katalysiert den ersten Schritt der CO2-Assimilierung. Trotz seiner bedeutenden Rolle, zeichnet sich Rubisco durch eine niedrige katalytische Geschwindigkeit aus. Außerdem, entsteht bei der Bindung von O2 anstatt CO2 ein toxisches Zwischenprodukt, welches in einem Prozess, genannt Photorespiration, aufbereitet wird. Da fast die gesamte Nahrungsmittelversorgung auf der Aktivität von Rubisco basiert, könnten schon kleine Verbesserungen in der Spezifität für CO2 zu einem großen Effekt in der Photosysntheserate und letztendlich größeren Ernteerträgen führen. In dieser Arbeit wurde versucht die Effizienz der Photosynthese zu verbessern, indem ein künstlicher CO2 konzentrierender Mechanismus aus einer Fusion von RbcL und einer Carboanhydrase (CA) gebildet wird. 
Als Vorversuch wurden je bCA1 und CAH3 an Rubiscos C- beziehungsweise N-Terminus mittels eines kleinen, flexiblen Linkers aus 5 Aminosäuren fusioniert. Anschließend wurden weitere Fusionsproteine zwischen dem C-Terminus von RbcL und bCA1/CAH3 mittels Linkern von 14, 23, 32 und 41 Aminosäuren Länge in Chloroplasten von Tabak eingebracht. Die entstandenen transplastomischen Pflanzen mit bCA1-Fusionen waren trotz ihres sehr langsamen Wachstums dazu fähig schon bei kurzen Linkern autotroph zu wachsen und geringe Mengen an Fusionsproteinen zu akkumulieren. Pflanzen mit CAH3 Fusionsproteinen hingegen waren nur mit längeren Linkern autotroph, zeigten aber dafür ähnliche Wachstumsraten zum Wildtyp bei Nutzung des längsten Linkers. Außerdem enthielten diese Pflanzen größere Mengen an Fusionsproteinen aber auch eine erhöhte Anreicherung von kleineren Abbauprodukten.
Bei den in dieser Arbeit gewählten CA als Fusionsprotein mit RbcL konnte im Vergleich mit katalytisch inaktiven Varianten kein Effekt auf die CO2-Assimilierung gefunden werden. Wie das autotroph Wachstum sowie die Gaswechsel- und in-vitro-Aktivitätsmessungen zeigen, haben die Fusionen allerdings nicht die katalytische Aktivität von Rubisco blockiert. Ebenso verhielt sich die Carboxylierungsrate von Rubisco und deren Spezifität für CO2 unverändert. Dies weist darauf hin, dass trotz Rubiscos Faltungs- oder Assemblierungsdefekten das Anfügen von 60-150 Aminosäure an den C-Terminus von RbcL nicht die katalytische Leistung des Enzyms beeinträchtigt. Im Gegenteil, die Wachstumsdefekte waren durch die geringe Menge an Rubisco begründet, vermutlich verursacht durch Defekte in der Faltung oder Assemblierung von RbcL. Schlussendlich konnten wichtige Erkenntnisse für zukünftige gentechnische Veränderungen von Rubisco gemacht werden: Fusionen mit dem C-Terminus von RbcL wurden besser toleriert als mit dem N-Terminus und längere Linker verringerten die von der Fusion ausgelösten Wachstumsdefekte.
KW  - Rubisco
KW  - fusion
Y1  - 2017
ER  - 
TY  - JOUR
A1  - Schwarte, Sandra
A1  - Tiedemann, Ralph
T1  - A Gene Duplication/Loss Event in the Ribulose-1,5-Bisphosphate-Carboxylase/Oxygenase (Rubisco) Small Subunit Gene Family among Accessions of Arabidopsis thaliana
JF  - Molecular biology and evolution
N2  - Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase; EC 4.1.1.39), the most abundant protein in nature, catalyzes the assimilation of CO(2) (worldwide about 10(11) t each year) by carboxylation of ribulose-1,5-bisphosphate. It is a hexadecamer consisting of eight large and eight small subunits. Although the Rubisco large subunit (rbcL) is encoded by a single gene on the multicopy chloroplast genome, the Rubisco small subunits (rbcS) are encoded by a family of nuclear genes. In Arabidopsis thaliana, the rbcS gene family comprises four members, that is, rbcS-1a, rbcS-1b, rbcS-2b, and rbcS-3b. We sequenced all Rubisco genes in 26 worldwide distributed A. thaliana accessions. In three of these accessions, we detected a gene duplication/loss event, where rbcS-1b was lost and substituted by a duplicate of rbcS-2b (called rbcS-2b*). By screening 74 additional accessions using a specific polymerase chain reaction assay, we detected five additional accessions with this duplication/loss event. In summary, we found the gene duplication/loss in 8 of 100 A. thaliana accessions, namely, Bch, Bu, Bur, Cvi, Fei, Lm, Sha, and Sorbo. We sequenced an about 1-kb promoter region for all Rubisco genes as well. This analysis revealed that the gene duplication/loss event was associated with promoter alterations (two insertions of 450 and 850 bp, one deletion of 730 bp) in rbcS-2b and a promoter deletion (2.3 kb) in rbcS-2b* in all eight affected accessions. The substitution of rbcS-1b by a duplicate of rbcS-2b (i.e., rbcS-2b*) might be caused by gene conversion. All four Rubisco genes evolve under purifying selection, as expected for central genes of the highly conserved photosystem of green plants. We inferred a single positive selected site, a tyrosine to aspartic acid substitution at position 72 in rbcS-1b. Exactly the same substitution compromises carboxylase activity in the cyanobacterium Anacystis nidulans. In A. thaliana, this substitution is associated with an inferred recombination. Functional implications of the substitution remain to be evaluated.
KW  - Arabidopsis thaliana
KW  - Arabidopsis lyrata
KW  - Rubisco
KW  - gene duplication
KW  - positive selection
Y1  - 2011
U6  - https://doi.org/10.1093/molbev/msr008
SN  - 0737-4038
VL  - 28
IS  - 6
SP  - 1861
EP  - 1876
PB  - Oxford Univ. Press
CY  - Oxford
ER  -