The dependency of red Rubisco on its cognate activase for enhancing plant photosynthesis and growth

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 117; no. 41; pp. 25890 - 25896
• Main Authors: Gunn, Laura H., Avila, Elena Martin, Birch, Rosemary, Whitney, Spencer M.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 13.10.2020
• Subjects: Algae; Assembly; Bacterial Proteins - chemistry; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Biological Sciences; Carbon dioxide; Carbon Dioxide - metabolism; Carbon dioxide fixation; carbon fixation; chloroplast transformation; Chloroplasts; Chloroplasts - metabolism; Coliforms; Compatibility; Directed evolution; E coli; Folding; Gene Expression; Genetic transformation; Kinetics; Metabolism; Nicotiana - chemistry; Nicotiana - genetics; Nicotiana - growth & development; Nicotiana - metabolism; Nuclear transformations; Photosynthesis; Plants, Genetically Modified - chemistry; Plants, Genetically Modified - genetics; Plants, Genetically Modified - growth & development; Plants, Genetically Modified - metabolism; Progeny; Protein folding; Proteins; Rhodobacter sphaeroides - enzymology; Rhodobacter sphaeroides - genetics; Rhodophyta; Ribulose-bisphosphate carboxylase; Ribulose-Bisphosphate Carboxylase - chemistry; Ribulose-Bisphosphate Carboxylase - genetics; Ribulose-Bisphosphate Carboxylase - metabolism; Rubisco activase; Tobacco; Transcription; Rubisco activase; chloroplast transformation; photosynthesis; carbon fixation
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.2011641117

Plant photosynthesis and growth are often limited by the activity of the CO₂-fixing enzyme Rubisco. The broad kinetic diversity of Rubisco in nature is accompanied by differences in the composition and compatibility of the ancillary proteins needed for its folding, assembly, and metabolic regulation. Variations in the protein folding needs of catalytically efficient red algae Rubisco prevent their production in plants. Here, we show this impediment does not extend to Rubisco from Rhodobacter sphaeroides (RsRubisco)—a red-type Rubisco able to assemble in plant chloroplasts. In transplastomic tobRsLS lines expressing a codon optimized Rs-rbcLS operon, the messenger RNA (mRNA) abundance was ∼25%of rbcL transcript and RsRubisco ∼40% the Rubisco content in WT tobacco. To mitigate the low activation status of RsRubisco in tobRsLS (∼23% sites active under ambient CO₂), the metabolic repair protein RsRca (Rs-activase) was introduced via nuclear transformation. RsRca production in the tobRsLS::X progeny matched endogenous tobacco Rca levels (∼1 μmol protomer·m²) and enhanced RsRubisco activation to 75% under elevated CO₂ (1%, vol/vol) growth. Accordingly, the rate of photosynthesis and growth in the tobRsLS::X lines were improved >twofold relative to tobRsLS. Other tobacco lines producing RsRubisco containing alternate diatom and red algae S-subunits were nonviable as CO₂-fixation rates (kcat c) were reduced >95%and CO₂/O₂ specificity impaired 30–50%. We show differences in hybrid and WT RsRubisco biogenesis in tobacco correlated with assembly in Escherichia coli advocating use of this bacterium to preevaluate the kinetic and chloroplast compatibility of engineered RsRubisco, an isoform amenable to directed evolution.