Functional biosynthesis of an allophycocyan beta subunit in Escherichia coli

Metabolic engineering of Escherichia coli for efficient biosynthesis of fluorescent phycobiliprotein

Background

Phycobiliproteins (PBPs) are light-harvesting protein found in cyanobacteria, red algae and the cryptomonads. They have been widely used as fluorescent labels in cytometry and immunofluorescence analysis. A number of PBPs has been produced in metabolically engineered Escherichia coli. However, the recombinant PBPs are incompletely chromophorylated, and the underlying mechanisms are not clear.

Results and discussion

In this work, a pathway for SLA-PEB [a fusion protein of streptavidin and allophycocyanin that covalently binds phycoerythrobilin (PEB)] biosynthesis in E. coli was constructed using a single-expression plasmid strategy. Compared with a previous E. coli strain transformed with dual plasmids, the E. coli strain transformed with a single plasmid showed increased plasmid stability and produced SLA-PEB with a higher chromophorylation ratio. To achieve full chromophorylation of SLA-PEB, directed evolution was employed to improve the catalytic performance of lyase CpcS. In addition, the catalytic abilities of heme oxygenases from different cyanobacteria were investigated based on biliverdin IXα and PEB accumulation. Upregulation of the heme biosynthetic pathway genes was also carried out to increase heme availability and PEB biosynthesis in E. coli. Fed-batch fermentation was conducted for the strain V5ALD, which produced recombinant SLA-PEB with a chromophorylation ratio of 96.7%.

Conclusion

In addition to reporting the highest chromophorylation ratio of recombinant PBPs to date, this work demonstrated strategies for improving the chromophorylation of recombinant protein, especially biliprotein with heme, or its derivatives as a prosthetic group.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1100-6) contains supplementary material, which is available to authorized users.

Efficient purification protocol for bioengineering allophycocyanin trimer with N-terminus Histag

Allophycocyanin plays a key role for the photon energy transfer from the phycobilisome to reaction center chlorophylls with high efficiency in cyanobacteria. Previously, the high soluble self-assembled bioengineering allophycocyanin trimer with N-terminus polyhistidine from Synechocystis sp. PCC 6803 had been successfully recombined and expressed in Escherichia coli strain. The standard protocol with immobilized metal-ion affinity chromatography with chelating transition metal ion (Ni2+) was used to purify the recombinant protein. Extensive optimization works were performed to obtain the desired protocol for high efficiency, low disassociation, simplicity and fitting for large-scale purification. In this study, a 33 full factorial response surface methodology was employed to optimize the varied factors such as pH of potassium phosphate (X1), NaCl concentration (X2), and imidazole concentration (X3). A maximum trimerization ratio (Y1) of approximate A650 nm/A620 nm at 1.024 was obtained at these optimum parameters. Further examinations, with absorbance spectra, fluorescence spectra and SDS-PAGE, confirmed the presence of bioengineering allophycocyanin trimer with highly trimeric form. All these results demonstrate that optimized protocol is efficient in purification of bioengineering allophycocyanin trimer with Histag.

Occurrence of a functionally stable photoharvesting single peptide allophycocyanin α-subunit (16.4 kDa) in the cyanobacterium Nostoc sp. R76DM

We report the occurrence of a functionally stable single peptide APC α-subunit in cyanobacterium Nostoc sp. R76DM.

Improving production of thermostable and fluorescent holo-β-allophycocyanin by metabolically engineered Escherichia coli using response surface methodology

A stable fluorescent holo-β-allophycocyanin (holo-ApcB) was produced by metabolically engineered Escherichia coli. The E. coli cells harbored two plasmids for expression of five genes that were involved in the holo-ApcB production. Response surface methodology was employed to investigate the individual and interactive effects of four variables, i.e., initial pH of culture medium, IPTG concentration, post-induction temperature, and induction start time, on holo-ApcB production by E. coli. The experimental results showed that the IPTG concentration, postinduction temperature, and induction start time had significant individual effects on holo-ApcB production. A significant interactive effect was also found between the initial pH of culture and induction start time. The maximum holo-ApcB production of 45.3 mg/L was predicted under the following optimized culture conditions: a postinduction temperature of 28.4°C, initial pH of culture of 7.3, IPTG concentration of 1.1 mM, and postinduction time of 66 min. Holo-ApcB production under the optimized culture conditions increased 5.8-fold, compared with that under the nonoptimized conditions. Response surface methodology proved to be a valuable tool for optimization of holo-ApcB production by metabolically engineered E. coli.

Biosynthesis of a stable allophycocyanin beta subunit in metabolically engineered Escherichia coli

Allophycocyanin (APC) is widely used as a fluorescent tag for fluorescence detection techniques. In this study, the apcB gene from a thermophilic cyanobacterium strain was cloned and ligated into an expression vector to construct a pathway for the biosynthesis of an allophycocyanin beta subunit (holo-apcBT) in Escherichia coli. Isopropyl β-d-1-thiogalactopyranoside induction successfully reconstituted holo-apcBT in E. coli. The recombinant holo-apcB showed spectroscopic properties similar to native APC. The stability and spectroscopic properties of this protein were then compared with another recombinant allophycocyanin beta subunit (holo-apcBM) whose apcB gene was cloned from mesophilic cyanobacterium. At high temperatures and during the course of storage, holo-apcBT was significantly more stable than holo-apcBM. In addition, holo-apcBT had an unexpectedly higher extinction coefficient and fluorescence quantum yield than holo-apcBM, suggesting that holo-apcBT would be a promising fluorescent tag and serve as a substitute for native APC.

Attachment of noncognate chromophores to CpcA of Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7002 by heterologous expression in Escherichia coli

Many cyanobacteria use brilliantly pigmented, multisubunit macromolecular structures known as phycobilisomes as antenna to enhance light harvesting for photosynthesis. Recent studies have defined the enzymes that synthesize phycobilin chromophores as well as many of the phycobilin lyase enzymes that attach these chromophores to their cognate apoproteins. The ability of the phycocyanin α-subunit (CpcA) to bind alternative linear tetrapyrrole chromophores was examined through the use of a heterologous expression system in Escherichia coli. E. coli strains produced phycocyanobilin, phytochromobilin, or phycoerythrobilin when they expressed 3Z-phycocyanobilin:ferredoxin oxidoreductase (PcyA), 3Z-phytochromobilin:ferredoxin oxidoreductase (HY2) from Arabidopsis thaliana, or phycoerythrobilin synthase (PebS) from the myovirus P-SSM4, respectively. CpcA from Synechocystis sp. PCC 6803 or Synechococcus sp. PCC 7002 was coexpressed in these strains with the phycocyanin α-subunit phycocyanobilin lyase, CpcE/CpcF, or the phycoerythrocyanin α-subunit phycocyanobilin isomerizing lyase, PecE/PecF, from Noctoc sp. PCC 7120. Both lyases were capable of attaching three different linear tetrapyrrole chromophores to CpcA; thus, up to six different CpcA variants, each with a unique chromophore, could be produced with this system. One of these chromophores, denoted phytoviolobilin, has not yet been observed naturally. The recombinant proteins had unexpected and potentially useful properties, which included very high fluorescence quantum yields and photochemical activity. Chimeric lyases PecE/CpcF and CpcE/PecF were used to show that the isomerizing activity that converts phycocyanobilin to phycoviolobilin resides with PecF and not PecE. Finally, spectroscopic properties of recombinant phycocyanin R-PCIII, in which the CpcA subunits carry a phycoerythrobilin chromophore, are described.

Biosynthesis of cyanobacterial phycobiliproteins in Escherichia coli: chromophorylation efficiency and specificity of all bilin lyases from Synechococcus sp. strain PCC 7002

Phycobiliproteins are water-soluble, light-harvesting proteins that are highly fluorescent due to linear tetrapyrrole chromophores, which makes them valuable as probes. Enzymes called bilin lyases usually attach these bilin chromophores to specific cysteine residues within the alpha and beta subunits via thioether linkages. A multiplasmid coexpression system was used to recreate the biosynthetic pathway for phycobiliproteins from the cyanobacterium Synechococcus sp. strain PCC 7002 in Escherichia coli. This system efficiently produced chromophorylated allophycocyanin (ApcA/ApcB) and alpha-phycocyanin with holoprotein yields ranging from 3 to 12 mg liter(-1) of culture. This heterologous expression system was used to demonstrate that the CpcS-I and CpcU proteins are both required to attach phycocyanobilin (PCB) to allophycocyanin subunits ApcD (alpha(AP-B)) and ApcF (beta(18)). The N-terminal, allophycocyanin-like domain of ApcE (L(CM)(99)) was produced in soluble form and was shown to have intrinsic bilin lyase activity. Lastly, this in vivo system was used to evaluate the efficiency of the bilin lyases for production of beta-phycocyanin.