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Title: Role of carboxysomes in cyanobacterial CO2 assimilation: CO2 concentrating mechanisms and metabolon implications

Many carbon‐fixing organisms have evolved CO2concentrating mechanisms (CCMs) to enhance the delivery of CO2to RuBisCO, while minimizing reactions with the competitive inhibitor, molecular O2. These distinct types of CCMs have been extensively studied using genetics, biochemistry, cell imaging, mass spectrometry, and metabolic flux analysis. Highlighted in this paper, the cyanobacterial CCM features a bacterial microcompartment (BMC) called ‘carboxysome’ in which RuBisCO is co‐encapsulated with the enzyme carbonic anhydrase (CA) within a semi‐permeable protein shell. The cyanobacterial CCM is capable of increasing CO2around RuBisCO, leading to one of the most efficient processes known for fixing ambient CO2. The carboxysome life cycle is dynamic and creates a unique subcellular environment that promotes activity of the Calvin–Benson (CB) cycle. The carboxysome may function within a larger cellular metabolon, physical association of functionally coupled proteins, to enhance metabolite channelling and carbon flux. In light of CCMs, synthetic biology approaches have been used to improve enzyme complex for CO2fixations. Research on CCM‐associated metabolons has also inspired biologists to engineer multi‐step pathways by providing anchoring points for enzyme cascades to channel intermediate metabolites towards valuable products.

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Journal Name:
Environmental Microbiology
Page Range / eLocation ID:
p. 219-228
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract Plain language summary

    Cyanobacteria are well known to fix atmospheric CO2into sugars using the enzyme Rubisco. Less appreciated are the carbon‐fixing abilities of proteobacteria with diverse metabolisms. Bacterial Rubisco is housed within organelles called carboxysomes that increase enzymatic efficiency. Here we show that proteobacterial carboxysomes are distributed in the cell by two proteins, McdA and McdB. McdA on the nucleoid interacts with McdB on carboxysomes to equidistantly space carboxysomes from one another, ensuring metabolic homeostasis and a proper inheritance of carboxysomes following cell division. This study illuminates how widespread carboxysome positioning systems are among diverse bacteria. Carboxysomes significantly contribute to global carbon fixation; therefore, understanding the spatial organization mechanism shared across the bacterial world is of great interest.

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  2. Summary

    Photosynthesis in C3 plants is limited by features of the carbon‐fixing enzyme Rubisco, which exhibits a low turnover rate and can react with O2instead ofCO2, leading to photorespiration. In cyanobacteria, bacterial microcompartments, known as carboxysomes, improve the efficiency of photosynthesis by concentratingCO2near the enzyme Rubisco. Cyanobacterial Rubisco enzymes are faster than those of C3 plants, though they have lower specificity towardCO2than the land plant enzyme. Replacement of land plant Rubisco by faster bacterial variants with lowerCO2specificity will improve photosynthesis only if a microcompartment capable of concentratingCO2can also be installed into the chloroplast. We review current information about cyanobacterial microcompartments and carbon‐concentrating mechanisms, plant transformation strategies, replacement of Rubisco in a model C3 plant with cyanobacterial Rubisco and progress toward synthesizing a carboxysome in chloroplasts.

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  3. Abstract

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  5. Summary

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