Abstract
BackgroundOxygen-evolving photoautotrophic organisms, like cyanobacteria, protect their photosynthetic machinery by a number of regulatory mechanisms, including alternative electron transfer pathways. Despite the importance in modulating the electron flux distribution between the photosystems, alternative electron transfer routes may compete with the solar-driven production of CO2-derived target chemicals in biotechnological systems under development. This work focused on engineered cyanobacterial Synechocystis sp. PCC 6803 strains, to explore possibilities to rescue excited electrons that would normally be lost to molecular oxygen by an alternative acceptor flavodiiron protein Flv1/3—an enzyme that is natively associated with transfer of electrons from PSI to O2, as part of an acclimation strategy towards varying environmental conditions.ResultsThe effects of Flv1/3 inactivation by flv3 deletion were studied in respect to three alternative end-products, sucrose, polyhydroxybutyrate and glycogen, while the photosynthetic gas fluxes were monitored by Membrane Inlet Mass Spectrometry (MIMS) to acquire information on cellular carbon uptake, and the production and consumption of O2. The results demonstrated that a significant proportion of the excited electrons derived from photosynthetic water cleavage was lost to molecular oxygen via Flv1/3 in cells grown under high CO2, especially under high light intensities. In flv3 deletion strains these electrons could be re-routed to increase the relative metabolic flux towards the monitored target products, but the carbon distribution and the overall efficiency were determined by the light conditions and the genetic composition of the respective pathways. At the same time, the total photosynthetic capacity of the Δflv3 strains was systematically reduced, and accompanied by upregulation of oxidative glycolytic metabolism in respect to controls with the native Flv1/3 background.ConclusionsThe observed metabolic changes and respective production profiles were proposedly linked with the lack of Flv1/3-mediated electron transfer, and the associated decrease in the intracellular ATP/NADPH ratio, which is bound to affect the metabolic carbon partitioning in the flv3-deficient cells. While the deletion of flv3 could offer a strategy for enhancing the photosynthetic production of desired chemicals in cyanobacteria under specified conditions, the engineered target pathways have to be carefully selected to align with the intracellular redox balance of the cells.
Highlights
Photosynthetic cyanobacteria have been considered as potential next-generation biotechnological hosts for the production of different carbon-based products, as part of the development of new carbon neutral industrial solutions to replace petroleum-derived chemicals in use [1, 2]
The objective of the study was to investigate whether the autotrophic production efficiency of specified end-metabolites can be improved in engineered cyanobacterial cells by the modification of endogenous regulatory systems, which are involved in the redistribution of photosynthetic electron fluxes when the light reactions and carbon fixation are not in balance
The focus was on a specific heterodimeric cyanobacterial Class C flavodiiron protein Flv1/3, which directs electrons accumulated in the photosynthetic electron transfer chain via photosystem (PS) I to molecular oxygen
Summary
Photosynthetic cyanobacteria have been considered as potential next-generation biotechnological hosts for the production of different carbon-based products, as part of the development of new carbon neutral industrial solutions to replace petroleum-derived chemicals in use [1, 2]. The critical advantage of such autotrophic production platforms would be the possibility to convert CO2 directly into the target metabolites by using photosynthetically captured solar radiation as the sole energy source, and cyanobacterial cells as biological catalysts This would decouple the production process from the dependence of biomass-based substrates, which is a key limitation in all currently existing large-scale biotechnological applications that use heterotrophic organisms as hosts. The focus was on a specific heterodimeric cyanobacterial Class C flavodiiron protein Flv1/3 (see review [7]), which directs electrons accumulated in the photosynthetic electron transfer chain via photosystem (PS) I to molecular oxygen In this so-called Mehler-like reaction [8, 9], Flv1/3 uses electrons which originate from the photosynthetic water-splitting in PSII and are thereafter directed via PSI to reduce O 2 to water (Fig. 1), without the formation of ROS [10]. PCC 6803 strains, to explore possibilities to rescue excited electrons that would normally be lost to molecular oxygen by an alternative acceptor flavodiiron protein Flv1/3—an enzyme that is natively associated with transfer of electrons from PSI to O 2, as part of an acclimation strategy towards varying environmental conditions
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