Over-expression of an electron transport protein OmcS provides sufficient NADH for d-lactate production in cyanobacterium

Abstract

Background An efficient supply of reducing equivalent is essential for chemicals production by engineered microbes. In phototrophic microbes, the NADPH generated from photosynthesis is the dominant form of reducing equivalent. However, most dehydrogenases prefer to utilize NADH as a cofactor. Thus, sufficient NADH supply is crucial to produce dehydrogenase-derived chemicals in cyanobacteria. Photosynthetic electron is the sole energy source and excess electrons are wasted in the light reactions of photosynthesis. Results Here we propose a novel strategy to direct the electrons to generate more ATP from light reactions to provide sufficient NADH for lactate production. To this end, we introduced an electron transport protein-encoding gene omcS into cyanobacterium Synechococcus elongatus UTEX 2973 and demonstrated that the introduced OmcS directs excess electrons from plastoquinone (PQ) to photosystem I (PSI) to stimulate cyclic electron transfer (CET). As a result, an approximately 30% increased intracellular ATP, 60% increased intracellular NADH concentrations and up to 60% increased biomass production with fourfold increased d-lactate production were achieved. Comparative transcriptome analysis showed upregulation of proteins involved in linear electron transfer (LET), CET, and downregulation of proteins involved in respiratory electron transfer (RET), giving hints to understand the increased levels of ATP and NADH. Conclusions This strategy provides a novel orthologous way to improve photosynthesis via enhancing CET and supply sufficient NADH for the photosynthetic production of chemicals.

Engineering Cyanobacterium with Transmembrane Electron Transfer Ability for Bioelectrochemical Nitrogen Fixation

“…A recent work expressed OmcS in S. elongatus UTEX 2973 showing that the soluble OmcS in the cytoplasm could act as a charge carrier directing electron flow from the quinone pool to PSI to stimulate cyclic electron transport. 66 A similar mechanism is likely here to aid electron transport to nitrogenase. However, considering that the CO 2 uptake rates, the biomass compositions, and the metabolic flux are very different between S. elongatus UTEX 2973 and S. elongatus PCC 7942, 67 more future studies are needed to unveil the role intracellular OmcS protein plays in the cellular electron transport chain inside S. elongatus PCC 7942.…”

“…A recent work expressed OmcS in S. elongatus UTEX 2973 showing that the soluble OmcS in the cytoplasm could act as a charge carrier directing electron flow from the quinone pool to PSI to stimulate cyclic electron transport . A similar mechanism is likely here to aid electron transport to nitrogenase.…”

A 300-fold conductivity increase in microbial cytochrome nanowires due to temperature-induced restructuring of hydrogen bonding networks

“…OmcS is essential for the reduction of iron oxides, an important electron acceptor in the native environment of G. sulfurreducens , as well during the early growth stages of electricity-producing biofilms in bioelectrochemical systems ( 49 ). In addition, artificially expressing cytochrome OmcS in photosynthetic cyanobacteria increased catalytic performance in a diversity of processes such as an increase in photocurrent by 9-fold ( 50 ), increased nitrogen fixation by 13-fold ( 51 ), and improved photosynthesis by increasing 60% biomass ( 52 ) compared to the wild-type cyanobacteria. These studies highlight the important role of OmcS in light-driven biocatalysis.…”

Synthetic Biology Approaches for Improving Chemical Production in Cyanobacteria

“…While efforts to improve RuBisCO have not been met with much success, current research has shifted focus towards rerouting metabolism to improve overall photosynthetic efficiency by focusing on key aspects of the Calvin-Benson cycle or the photosynthetic electron transport chain (PETC). One strategy used to harness the excess energy being lost by the PETC in cyanobacteria involved overexpressing the protein OmcS ( Meng et al, 2021 ). This strategy coupled the excess electrons from the PETC to NADH production and was shown to increase intracellular ATP and NADH allowing for a fourfold improvement of D -lactate production in the cyanobacterium Synechococcus elongatus UTEX 2973 (hereon 2973) ( Meng et al, 2021 ).…”

“…One strategy used to harness the excess energy being lost by the PETC in cyanobacteria involved overexpressing the protein OmcS ( Meng et al, 2021 ). This strategy coupled the excess electrons from the PETC to NADH production and was shown to increase intracellular ATP and NADH allowing for a fourfold improvement of D -lactate production in the cyanobacterium Synechococcus elongatus UTEX 2973 (hereon 2973) ( Meng et al, 2021 ).…”