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1. The McdAB system positions α‐carboxysomes in proteobacteria

   https://doi.org/10.1111/mmi.14708  

   MacCready, Joshua S. ; Tran, Lisa ; Basalla, Joseph L. ; Hakim, Pusparanee ; Vecchiarelli, Anthony G. ( March 2021 , Molecular Microbiology)

   Carboxysomes are protein‐based organelles essential for carbon fixation in cyanobacteria and proteobacteria. Previously, we showed that the cyanobacterial nucleoid is used to equally space out β‐carboxysomes across cell lengths by a two‐component system (McdAB) in the model cyanobacteriumSynechococcus elongatusPCC 7942. More recently, we found that McdAB systems are widespread among β‐cyanobacteria, which possess β‐carboxysomes, but are absent in α‐cyanobacteria, which possess structurally and phyletically distinct α‐carboxysomes. Cyanobacterial α‐carboxysomes are thought to have arisen in proteobacteria and then horizontally transferred into cyanobacteria, which suggests that α‐carboxysomes in proteobacteria may also lack the McdAB system. Here, using the model chemoautotrophic proteobacteriumHalothiobacillus neapolitanus, we show that a McdAB system distinct from that of β‐cyanobacteria operates to position α‐carboxysomes across cell lengths. We further show that this system is widespread among α‐carboxysome‐containing proteobacteria and that cyanobacteria likely inherited an α‐carboxysome operon from a proteobacterium lacking themcdABlocus. These results demonstrate that McdAB is a cross‐phylum two‐component system necessary for positioning both α‐ and β‐carboxysomes. The findings have further implications for understanding the positioning of other protein‐based bacterial organelles involved in diverse metabolic processes.

   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. Towards engineering carboxysomes into C3 plants

   https://doi.org/10.1111/tpj.13139  

   Hanson, Maureen R. ; Lin, Myat T. ; Carmo‐Silva, A. Elizabete ; Parry, Martin A. J. ( June 2016 , The Plant Journal)

   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. Effects of RuBisCO and CO 2 concentration on cyanobacterial growth and carbon isotope fractionation

   https://doi.org/10.1111/gbi.12543  

   Garcia, Amanda K. ; Kędzior, Mateusz ; Taton, Arnaud ; Li, Meng ; Young, Jodi N. ; Kaçar, Betül ( May 2023 , Geobiology)

   Carbon isotope biosignatures preserved in the Precambrian geologic record are primarily interpreted to reflect ancient cyanobacterial carbon fixation catalyzed by Form I RuBisCO enzymes. The average range of isotopic biosignatures generally follows that produced by extant cyanobacteria. However, this observation is difficult to reconcile with several environmental (e.g., temperature, pH, and CO₂concentrations), molecular, and physiological factors that likely would have differed during the Precambrian and can produce fractionation variability in contemporary organisms that meets or exceeds that observed in the geologic record. To test a specific range of genetic and environmental factors that may impact ancient carbon isotope biosignatures, we engineered a mutant strain of the model cyanobacteriumSynechococcus elongatusPCC 7942 that overexpresses RuBisCO across varying atmospheric CO₂concentrations. We hypothesized that changes in RuBisCO expression would impact the net rates of intracellular CO₂fixation versus CO₂supply, and thus whole‐cell carbon isotope discrimination. In particular, we investigated the impacts of RuBisCO overexpression under changing CO₂concentrations on both carbon isotope biosignatures and cyanobacterial physiology, including cell growth and oxygen evolution rates. We found that an increased pool of active RuBisCO does not significantly affect the¹³C/¹²C isotopic discrimination (ε_p) at all tested CO₂concentrations, yielding ε_pof ≈ 23‰ for both wild‐type and mutant strains at elevated CO₂. We therefore suggest that expected variation in cyanobacterial RuBisCO expression patterns should not confound carbon isotope biosignature interpretation. A deeper understanding of environmental, evolutionary, and intracellular factors that impact cyanobacterial physiology and isotope discrimination is crucial for reconciling microbially driven carbon biosignatures with those preserved in the geologic record.

    

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4. Prospects for Engineering Biophysical CO2 Concentrating Mechanisms into Land Plants to Enhance Yields

   https://doi.org/10.1146/annurev-arplant-081519-040100  

   Hennacy, Jessica H ; Jonikas, Martin C ( March 2020 , Annual review of plant biology)

   Although cyanobacteria and algae represent a small fraction of the biomass of all primary producers, their photosynthetic activity accounts for roughly half of the daily CO2 fixation that occurs on Earth. These microorganisms are able to accomplish this feat by enhancing the activity of the CO2-fixing enzyme Rubisco using biophysical CO2 concentrating mechanisms (CCMs). Biophysical CCMs operate by concentrating bicarbonate and converting it into CO2 in a compartment that houses Rubisco (in contrast with other CCMs that concentrate CO2 via an organic intermediate, such as malate in the case of C4 CCMs). This activity provides Rubisco with a high concentration of its substrate, thereby increasing its reaction rate. The genetic engineering of a biophysical CCM into land plants is being pursued as a strategy to increase crop yields. This review focuses on the progress toward understanding the molecular components of cyanobacterial and algal CCMs, as well as recent advances toward engineering these components into land plants. 

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5. Isotope discrimination by form IC RubisCO from Ralstonia eutropha and Rhodobacter sphaeroides , metabolically versatile members of ‘ Proteobacteria ’ from aquatic and soil habitats

   https://doi.org/10.1111/1462-2920.14423  

   Thomas, Phaedra J. ; Boller, Amanda J. ; Satagopan, Sriram ; Tabita, F. Robert ; Cavanaugh, Colleen M. ; Scott, Kathleen M. ( November 2018 , Environmental Microbiology)

   RubisCO, the CO₂fixing enzyme of the Calvin–Benson–Bassham (CBB) cycle, is responsible for the majority of carbon fixation on Earth. RubisCO fixes¹²CO₂faster than¹³CO₂resulting in¹³C‐depleted biomass, enabling the use of δ¹³C values to trace CBB activity in contemporary and ancient environments. Enzymatic fractionation is expressed as anεvalue, and is routinely used in modelling, for example, the global carbon cycle and climate change, and for interpreting trophic interactions. Although values for spinach RubisCO (ε= ~29‰) have routinely been used in such efforts, there are five different forms of RubisCO utilized by diverse photolithoautotrophs and chemolithoautotrophs andεvalues, now known for four forms (IA, B, D and II), vary substantially withε =11‰ to 27‰. Given the importance ofεvalues in δ¹³C evaluation, we measured enzymatic fractionation of the fifth form, form IC RubisCO, which is found widely in aquatic and terrestrial environments. Values were determined for two model organisms, the ‘Proteobacteria’ Ralstonia eutropha (ε= 19.0‰) andRhodobacter sphaeroides(ε =22.4‰). It is apparent from these measurements that all RubisCO forms measured to date discriminate less than commonly assumed based on spinach, and that enzymeεvalues must be considered when interpreting and modelling variability of δ¹³C values in nature.

    

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