A phycocyanin-deficient mutant of synechocystis PCC 6714 with a single-base substitution upstream of the cpc operon

Production of thermostable phycocyanin in a mesophilic cyanobacterium

Phycocyanin (PC) is a soluble phycobiliprotein found within the light-harvesting phycobilisome complex of cyanobacteria and red algae, and is considered a high-value product due to its brilliant blue colour and fluorescent properties. However, commercially available PC has a relatively low temperature stability. Thermophilic species produce more thermostable variants of PC, but are challenging and energetically expensive to cultivate. Here, we show that the PC operon from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 (cpcBACD) is functional in the mesophile Synechocystis sp. PCC 6803. Expression of cpcBACD in an 'Olive' mutant strain of Synechocystis lacking endogenous PC resulted in high yields of thermostable PC (112 ± 1 mg g^-1 DW) comparable to that of endogenous PC in wild-type cells. Heterologous PC also improved the growth of the Olive mutant, which was further supported by evidence of a functional interaction with the endogenous allophycocyanin core of the phycobilisome complex. The thermostability properties of the heterologous PC were comparable to those of PC from T. elongatus, and could be purified from the Olive mutant using a low-cost heat treatment method. Finally, we developed a scalable model to calculate the energetic benefits of producing PC from T. elongatus in Synechocystis cultures. Our model showed that the higher yields and lower cultivation temperatures of Synechocystis resulted in a 3.5-fold increase in energy efficiency compared to T. elongatus, indicating that producing thermostable PC in non-native hosts is a cost-effective strategy for scaling to commercial production.

Rewiring of Cyanobacterial Metabolism for Hydrogen Production: Synthetic Biology Approaches and Challenges

With the demand for renewable energy growing, hydrogen (H₂) is becoming an attractive energy carrier. Developing H₂ production technologies with near-net zero carbon emissions is a major challenge for the "H₂ economy." Certain cyanobacteria inherently possess enzymes, nitrogenases, and bidirectional hydrogenases that are capable of H₂ evolution using sunlight, making them ideal cell factories for photocatalytic conversion of water to H₂. With the advances in synthetic biology, cyanobacteria are currently being developed as a "plug and play" chassis to produce H₂. This chapter describes the metabolic pathways involved and the theoretical limits to cyanobacterial H₂ production and summarizes the metabolic engineering technologies pursued.

Regulation of β-phellandrene synthase gene expression, recombinant protein accumulation, and monoterpene hydrocarbons production in Synechocystis transformants

Successful application of the photosynthesis-to-fuels approach requires a high product-to-biomass carbon-partitioning ratio. The work points to the limiting amounts of heterologous terpene synthase in cyanobacteria as a potential barrier in the yield of terpene hydrocarbons via photosynthesis. Cyanobacteria like Synechocystis sp. can be exploited as platforms in a photosynthesis-to-fuels process for the generation of terpene hydrocarbons. Successful application of this concept requires maximizing photosynthesis and attaining a high endogenous carbon partitioning toward the desirable product. The work addressed the question of the regulation of β-phellandrene synthase transgene expression in relation to product yield from the terpenoid biosynthetic pathway of cyanobacteria. The choice of strong alternative transcriptional and translational cis-regulatory elements and the choice of the Synechocystis genomic DNA loci for transgene insertion were investigated. Specifically, the β-phellandrene synthase transgene was expressed under the control of the endogenous psbA2 promoter, or under the control of the Ptrc promoter from Escherichia coli with the translation initiation region of highly expressed gene 10 from bacteriophage T7. These heterologous elements allowed for constitutive transgene expression. In addition, the β-phellandrene synthase construct was directed to replace the Synechocystis cpc operon, encoding the peripheral phycocyanin rods of the phycobilisome antenna. Results showed that a 4-fold increase in the cellular content of the β-phellandrene synthase was accompanied by a 22-fold increase in β-phellandrene yield, suggesting limitations in rate and yield by the amount of the transgenic enzyme. The work points to the limiting amount of transgenic terpene synthases as a potential barrier in the heterologous generation of terpene products via the process of photosynthesis.

Characterization and responses to environmental cues of a photosynthetic antenna-deficient mutant of the filamentous cyanobacterium Anabaena sp. PCC 7120

The cyanobacterial phycobilisome (PBS) is a giant pigment-protein complex which harvests light energy for photosynthesis and comprises two structures: a core and peripheral rods. Most studies on PBS structure and function are based on mutants of unicellular strains. In this report, we describe the phenotypic and genetic characterization of a transposon mutant of the filamentous Anabaena sp. strain PCC 7120, denoted LC1, which cannot synthesize the phycobiliprotein phycocyanin (PC), the main component of the rods; in this mutant, the transposon had inserted into the cpcB gene (orf alr0528) which putatively encodes PC-β chain. Mutant LC1 was able to synthesize phycoerythrocyanin (PEC), a phycobiliprotein (PBP) located at the terminal region of the rods; but in the absence of PC, PEC did not attach to the PBSs that only retained the allophycocyanin (APC) core; ferredoxin: NADP+-oxidoreductase (FNR) that is associated with the PBS in the wild type, was not found in isolated PBSs from LC1. The performance of the mutant exposed to different environmental conditions was evaluated. The mutant phenotype was successfully complemented by cloning and transfer of the wild type complete cpc operon to mutant LC1. Interestingly, LC1 compensated its mutation by significantly increasing the number of its core-PBS and the effective quantum yield of photosystem II (PSII) photochemistry; this feature suggests a more efficient energy conversion in the mutant which may be useful for biotechnological applications.

Biochemical and genetic engineering strategies to enhance hydrogen production in photosynthetic algae and cyanobacteria

As an energy carrier, hydrogen gas is a promising substitute to carbonaceous fuels owing to its superb conversion efficiency, non-polluting nature, and high energy content. At present, hydrogen is predominately synthesized via chemical reformation of fossil fuels. While various biological methods have been extensively explored, none of them is justified as economically feasible. A sustainable platform for biological production of hydrogen will certainly impact the biofuel market. Among a selection of biological systems, algae and cyanobacteria have garnered major interests as potential cell factories for hydrogen production. In conjunction with photosynthesis, these organisms utilize inexpensive inorganic substrates and solar energy for simultaneous biosynthesis and hydrogen evolution. However, the hydrogen yield associated with these organisms remains far too low to compete with the existing chemical systems. This article reviews recent advances of biochemical, bioprocess, and genetic engineering strategies in circumventing technological limitations to hopefully improve the applicative potential of these photosynthetic hydrogen production systems.

Analysis on the factors affecting start-up intensity in the upstream sequence of phycocyanin β subunit gene from Arthrospira platensis by site-directed mutagenesis

Six promoters in the 419 bp upstream sequence of the phycocyanin beta subunit gene of Arthrospira platensis FACHB341 have been previously cloned. Site-directed mutagenesis has now been used to introduce mutations in the -10 and -35 boxes of promoter 3, -10 box of promoter 4, and -35 box of promoter 6. The expression level of green fluorescent protein gene was measured by flow cytometry. Results showed that the effects of site-directed mutagenesis in different promoters were dissimilar: some increased and some declined.

A suppressor mutation in the alpha-phycocyanin gene in the light/glucose-sensitive phenotype of the psbK-disruptant of the cyanobacterium Synechocystis sp. PCC 6803

psbK encodes a small transmembrane component of PSII. Here we report that the psbK-disruptant of Synechocystis sp. PCC 6803 cannot survive under photomixotrophic conditions of light and glucose after transient growth, while the wild type is able to grow. A spontaneous yellow-green mutant that recovered the sustained growth under the same conditions was isolated from the psbK-disruptant. Instead of recovery, the mutant largely lost photoautotrophic growth. By phenotype complementation, the mutation was identified in cpcA as a sequence replacement with a close downstream segment, generating an inverted repeat of 23 bp. The mutant phenotype was characterized by (i) the complete loss of alpha- and beta-phycocyanin; (ii) increased accumulation of PSII; and (iii) greatly reduced transcripts harboring cpcA in abundance and in size. The inverted repeat generated in cpcA probably led to the early termination of transcription. A possible mechanism for such a mutation is discussed.

Thymine at -5 is crucial for cpc promoter activity of Synechocystis sp. strain PCC 6714

The levels of transcripts of the cpc operon were highly reduced in a PD-1 mutant of cyanobacterium Synechocystis sp. strain PCC 6714. This was due to a substitution of C for T that occurred at 5 bp upstream of the transcription initiation site of the cpc operon. Any substitution for T at the -5 position drastically reduced both in vivo and in vitro promoter activity in cyanobacterium Synechococcus sp. strain PCC 7942 but not the in vivo activity in Escherichia coli. This suggests that the requirement of -5T appears to be specific for a cyanobacterial RNA polymerase-promoter combination.