Effects of inactivation of the cyAbrB2 transcription factor together with glycogen synthesis on cellular metabolism and free fatty acid production in the cyanobacterium Synechocystis sp. PCC 6803

Tailoring regulatory components for metabolic engineering in cyanobacteria

The looming climate crisis has prompted an ever-growing interest in cyanobacteria due to their potential as sustainable production platforms for the synthesis of energy carriers and value-added chemicals from CO2 and sunlight. Nonetheless, cyanobacteria are yet to compete with heterotrophic systems in terms of space-time yields and consequently production costs. One major drawback leading to the low production performance observed in cyanobacteria is the limited ability to utilize the full capacity of the photosynthetic apparatus and its associated systems, i.e. CO2 fixation and the directly connected metabolism. In this review, novel insights into various levels of metabolic regulation of cyanobacteria are discussed, including the potential of targeting these regulatory mechanisms to create a chassis with a phenotype favorable for photoautotrophic production. Compared to conventional metabolic engineering approaches, minor perturbations of regulatory mechanisms can have wide-ranging effects.

Production of Fatty Acids and Derivatives Using Cyanobacteria

Fatty acids and their derivatives are highly valuable chemicals that can be produced through chemical or enzymatic processes using plant lipids. This may compete with human food sources. Therefore, there has been an urge to create a new method for synthesizing these chemicals. One approach is to use microbial cells, specifically cyanobacteria, as a factory platform. Engineering may need to be implemented in order to allow a cost-competitive production and to enable a production of a variety of different fatty acids and derivatives. In this chapter, we explain in details the importance of fatty acids and their derivatives, including fatty aldehydes, fatty alcohols, hydrocarbons, fatty acid methyl esters, and hydroxy fatty acids. The production of these chemicals using cyanobacterial native metabolisms together with strategies to engineer them are also explained. Moreover, recent examples of fatty acid and fatty acid derivative production from engineered cyanobacteria are gathered and reported. Commercial opportunities to manufacture fatty acids and derivatives are also discussed in this chapter. Altogether, it is clear that fatty acids and their derivatives are important chemicals, and with recent advancements in genetic engineering, a cyanobacterial platform for bio-based production is feasible. However, there are regulations and guidelines in place for the use of genetically modified organisms (GMOs) and some further developments are still needed before commercialization can be reached.

Lethality caused by ADP-glucose accumulation is suppressed by salt-induced carbon flux redirection in cyanobacteria

Cyanobacteria are widely distributed photosynthetic organisms. During the day they store carbon, mainly as glycogen, to provide the energy and carbon source they require for maintenance during the night. Here, we generate a mutant strain of the freshwater cyanobacterium Synechocystis sp. PCC 6803 lacking both glycogen synthases. This mutant has a lethal phenotype due to massive accumulation of ADP-glucose, the substrate of glycogen synthases. This accumulation leads to alterations in its photosynthetic capacity and a dramatic decrease in the adenylate energy charge of the cell to values as low as 0.1. Lack of ADP-glucose pyrophosphorylase, the enzyme responsible for ADP-glucose synthesis, or reintroduction of any of the glycogen synthases abolishes the lethal phenotype. Viability of the glycogen synthase mutant is also fully recovered in NaCl-supplemented medium, which redirects the surplus of ADP-glucose to synthesize the osmolite glucosylglycerol. This alternative metabolic sink also suppresses phenotypes associated with the defective response to nitrogen deprivation characteristic of glycogen-less mutants, restoring the capacity to degrade phycobiliproteins. Thus, our system is an excellent example of how inadequate management of the adenine nucleotide pools results in a lethal phenotype, and the influence of metabolic carbon flux in cell viability and fitness.

Simultaneous increases in the levels of compatible solutes by cost-effective cultivation of Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803, a cyanobacterium widely used for basic research, is often cultivated in a synthetic medium, BG-11, in the presence of 4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid (HEPES) or 2-[[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]amino]ethanesulfonic acid buffer. Owing to the high cost of HEPES buffer (96.9% of the total cost of BG-11 medium), the biotechnological application of BG-11 is limited. In this study, we cultured Synechocystis sp. PCC 6803 cells in BG-11 medium without HEPES buffer and examined the effects on the primary metabolism. Synechocystis sp. PCC 6803 cells could grow in BG-11 medium without HEPES buffer after adjusting for nitrogen sources and light intensity; the production rate reached 0.54 g cell dry weight·L^-1 ·day^-1 , exceeding that of commercial cyanobacteria and Synechocystis sp. PCC 6803 cells cultivated under other conditions. The exclusion of HEPES buffer markedly altered the metabolites in the central carbon metabolism; particularly, the levels of compatible solutes, such as sucrose, glucosylglycerol, and glutamate were increased. Although the accumulation of sucrose and glucosylglycerol under high salt conditions is antagonistic to each other, these metabolites accumulated simultaneously in cells grown in the cost-effective medium. Because these metabolites are used in industrial feedstocks, our results reveal the importance of medium composition for the production of metabolites using cyanobacteria.

Light-inducible expression of translation factor EF-Tu during acclimation to strong light enhances the repair of photosystem II

Photoinhibition of photosystem II (PSII) is mitigated via enhancement of the repair of PSII in photosynthetic organisms that have acclimated to strong light, but the mechanism responsible for such acclimation of PSII repair remains unknown. We report here that an elevated level of translation factor EF-Tu during the acclimation of Synechocystis to strong light plays a critical role in the enhancement of PSII repair via acceleration of the synthesis de novo of proteins that are required for PSII repair, such as the D1 protein. We propose a mechanism for protection of PSII from photoinhibition that becomes operative during acclimation to strong light and appears to contribute to the ability of photosynthetic organisms to tolerate strong light.

In photosynthetic organisms, the repair of photosystem II (PSII) is enhanced after acclimation to strong light, with the resultant mitigation of photoinhibition of PSII. We previously reported that oxidation of translation elongation factor EF-Tu, which delivers aminoacyl-tRNA to the ribosome, depresses the repair of PSII in the cyanobacterium Synechocystis sp. PCC 6803. In the present study, we investigated the role of EF-Tu in the repair of PSII after acclimation of Synechocystis to strong light. In cells that had been grown under strong light, both the repair of PSII and the synthesis of proteins de novo were enhanced under strong light, with the resultant mitigation of photoinhibition of PSII. Moreover, levels of EF-Tu were elevated, whereas levels of other components of the translation machinery, such as translation factor EF-G and ribosomal proteins L2 and S12, did not change significantly. The expression of the gene for EF-Tu was induced by light, as monitored at the transcriptional level. Elevation of the level of EF-Tu was strongly correlated with the subsequent enhancement of PSII repair in cells that had been grown under light at various intensities. Furthermore, overexpression of EF-Tu in Synechocystis enhanced protein synthesis and PSII repair under strong light, even after cell culture under nonacclimating conditions. These observations suggest that elevation of the level of EF-Tu might be a critical factor in enhancing the capacity for repair of PSII that develops during acclimation to strong light.