Abstract
BackgroundAn 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.ResultsHere 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.ConclusionsThis strategy provides a novel orthologous way to improve photosynthesis via enhancing CET and supply sufficient NADH for the photosynthetic production of chemicals.
Highlights
Introduction ofOuter membrane cytochrome S (OmcS) upregulates the expression of genes encoding key enzymes of photosynthesis To gain insights into the mechanisms how introducing OmcS augments photosynthetic electron transfer to generate more ATP and contributes to providing sufficient NADH, we analyzed the transcriptome of d-lactate production strains with or without OmcS sampled at two time points during rapid d-lactate production period (Figs. 1 and 4b)
As the molecules of NADPH generated from photosynthesis are much more than the NADH generated from catabolism, NADPH is the dominant form of reducing equivalent in cyanobacteria, and the NADH pool in cyanobacteria is lower than its NADPH pool under photoautotrophic conditions [8,9,10,11]
Experiment design and introducing OmcS As already illustrated, compared with the NADPH directly generated from light energy, the NADH generated from the catabolic metabolism of organic carbon is the minor form of reducing equivalent in cyanobacteria
Summary
Introduction ofOmcS upregulates the expression of genes encoding key enzymes of photosynthesis To gain insights into the mechanisms how introducing OmcS augments photosynthetic electron transfer to generate more ATP and contributes to providing sufficient NADH, we analyzed the transcriptome of d-lactate production strains with or without OmcS sampled at two time points during rapid d-lactate production period (Figs. 1 and 4b). Compared to strain Syn2973-Ldh, most subunits of AP, PC/PEC and PE, especially ApcE and ApcD subunits of AP, the terminal energy emitters in the PBS core, which funnels light energy directly to PSII and PSI [43] in Syn2973-LdhOmcS strain, were significantly upregulated (Fig. 1, Additional file 1: Table S2). This indicates both PSII and PSI can absorb more light. Cytb6f and PC involved in photosynthetic electron transport and ferredoxinNADP+ reductase (FNR) involved in NADPH formation were upregulated (Fig. 1, Additional file 1: Table S3) This further supported the increased photosynthetic activity of PSII and PSI in Syn2973-LdhOmcS strain. NADH-dependent enzymes are less active than NADPH-dependent ones in cyanobacteria due to a shortage of NADH [12,13,14]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
