Project description:The slow kinetics and poor substrate specificity of the key photosynthetic CO2-fixing enzyme Rubisco have prompted the repeated evolution of Rubisco containing compartments known as pyrenoids in diverse algal lineages and carboxysomes in prokaryotes. Inside these compartments actively transported bicarbonate is converted into CO2 gas, which saturates the carboxylase with its substrate. Using co-immunoprecipitation experiments in Phaeodactylum tricornutum we have identified the Rubisco linker protein PYCO1. Similar to the green algal Rubisco linker protein EPYC1, PYCO1 is intrinsically disordered, possesses repeats and is positively charged at physiological pH. However, it possesses no sequence similarity to EPYC1, as expected for convergent evolution of a red Rubisco containing pyrenoid. Fluorescent PYCO1 fusion proteins localize as a rod shaped structure in the diatom chloroplast, consistent with the shape of the pyrenoid defined by transmission electron microscopy. To test the hypothesis that PYCO1 is the diatom pyrenoid scaffold we produced pure protein in Escherichia coli. Recombinant PYCO1 protein undergoes homotypic liquid liquid phase separation in a salt dependent manner. Diatom Rubisco specifically partitions into PYCO1 condensates. Heterotypic PYCO1-Rubisco condensates can bind up to three Rubisco hexadecamers per PYCO1 protein. Rubisco carboxylase function is unaffected in the condensates. PYCO1 is highly mobile in homotypic condensates. In contrast PYCO1 condensates saturated with diatom Rubisco have greatly reduced dynamics, with both PYCO1 and Rubisco becoming immobile. Consistently, FRAP experiments indicate that PYCO1 is not mobile in vivo. A combination of Cryo-electron microscopy and site-directed mutagenesis data show that the KWSP motif found in PYCO1 repeats binds to small subunits at the entrance of the Rubisco hexadecamer’s solvent channel. Analysis of mutant PYCO1 proteins show that both the “KWSP” tryptophan and another repeating tyrosine are essential for homotypic phase separation. We speculate that the unusual material properties of the PYCO1-Rubisco condensate are necessary to support the unusual non-spherical shape of the Phaeodactylum pyrenoid. Careful characterization of multiple diverse Rubisco condensates will strengthen translational approaches aiming to introduce pyrenoids and other metabolic condensates into new host organisms.

Project description:Background: Anaerobic Saccharomyces cerevisiae cultures require glycerol formation to re-oxidize NADH formed in biosynthetic processes. Introduction of the Calvin-cycle enzymes phosphoribulokinase (PRK) and ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCO) has been shown to couple re-oxidation of biosynthetic NADH to ethanol production and improve ethanol yield on sugar in fast-growing batch cultures. Since growth rates in industrial ethanol-production processes are not constant, performance of engineered strains was studied in slow-growing cultures. Results: In slow-growing anaerobic chemostat cultures (D = 0.05 h-1), an engineered PRK-RuBisCO strain produced 80-fold more acetaldehyde and 30-fold more acetate than a reference strain. This observation suggested an imbalance between in vivo activities of PRK-RuBisCO and formation of NADH in biosynthesis. Lowering the copy number of the RuBisCO-encoding cbbm expression cassette from 15 to 2 reduced acetaldehyde and acetate yields by 67% and 29%, respectively. Additional C-terminal fusion of a 19 amino-acid tag to PRK reduced its protein level by 13-fold while acetaldehyde and acetate production decreased by 94% and 61%, respectively, relative to the 15x cbbm strain. These modifications did not affect glycerol production at 0.05 h-1 but caused a 4.6 fold higher glycerol production per amount of biomass in fast-growing (0.29 h-1) anaerobic batch cultures than observed for the 15x cbbm strain. In another strategy, the promoter of ANB1, whose transcript level positively correlated with growth rate, was used to control PRK synthesis in a 2x cbbm strain. At 0.05 h-1, this strategy reduced acetaldehyde and acetate production by 79% and 40%, respectively, relative to the 15x cbbm strain, without affecting glycerol yield. The maximum growth rate of the resulting strain equalled that of the reference strain, while its glycerol yield was 72% lower. Conclusions: Acetaldehyde and acetate formation by slow-growing cultures of engineered S. cerevisiae strains carrying a PRK-RuBisCO bypass of yeast glycolysis was attributed to an in vivo overcapacity of PRK and RuBisCO. Reducing the capacity of PRK and/or RuBisCO was shown to mitigate this undesirable byproduct formation. Use of a growth-rate-dependent promoter for PRK expression highlighted the potential of modulating gene expression in engineered strains to respond to growth-rate dynamics in industrial batch processes.

Dataset Information

Rubisco

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure

Tweets