Display of organophosphorus hydrolase on the cyanobacterial cell surface using synechococcus outer membrane protein a as an anchoring motif

Population-level heterogeneity complicates utilization of Synechococcus elongatus PCC 7942 surface display platforms

Surface display technologies have been primarily developed for heterotrophic microbes, leaving photosynthetic counterparts like cyanobacteria with limited molecular tools. Here, we expanded upon surface display systems in Synechococcus elongatus PCC 7942 by modifying two outer-membrane proteins, SomA and Intimin, to display tags ( e.g. , SpyTag) to mediate physical interactions of living cyanobacteria with other biotic and abiotic targets. While re-engineered SomA constructs successfully translocated to the cell surface and could bind to compatible ligands, the efficacy of the best-performing designs was limited by a poorly-understood heterogeneity in the accessibility of the tags in living cells, resulting in low attachment penetrance.

Recent developments in the production and utilization of photosynthetic microorganisms for food applications

The growing use of photosynthetic microorganisms for food and food-related applications is driving related biotechnology research forward. Increasing consumer acceptance, high sustainability, demand of eco-friendly sources for food, and considerable global economic concern are among the main factors to enhance the focus on the novel foods. In the cases of not toxic strains, photosynthetic microorganisms not only provide a source of sustainable nutrients but are also potentially healthy. Several published studies showed that microalgae are sources of accessible protein and fatty acids. More than 400 manuscripts were published per year in the last 4 years. Furthermore, industrial approaches utilizing these microorganisms are resulting in new jobs and services. This is in line with the global strategy for bioeconomy that aims to support sustainable development of bio-based sectors. Despite the recognized potential of the microalgal biomass value chain, significant knowledge gaps still exist especially regarding their optimized production and utilization. This review highlights the potential of microalgae and cyanobacteria for food and food-related applications as well as their market size. The chosen topics also include advanced production as mixed microbial communities, production of high-value biomolecules, photoproduction of terpenoid flavoring compounds, their utilization for sustainable agriculture, application as source of nutrients in space, and a comparison with heterotrophic microorganisms like yeast to better evaluate their advantages over existing nutrient sources. This comprehensive assessment should stimulate further interest in this highly relevant research topic.

A safety type of genetically engineered bacterium that degrades chemical pesticides

Chemical pesticides are used widely and their residues are found in the environment. Pesticide pollution has become a global problem. To find an economical, effective and safety way to degrade residues of pesticides in environment, we constructed a genetically engineered bacterium (GEB) having the ability to degrade pesticides, emit green fluorescence and has a containment system by using a dual plasmid expression system. One plasmid contains the genes of enhanced green fluorescent protein (EGFP) and carboxylesterase B1 (CarE B1), which were cloned downstream of lambda P_L promoter and expressed constitutively. The gene of CarE B1 encodes an insect-detoxifying enzyme possessing the degradability to organochloride pesticides, organophosphorus pesticides, carbamates, and pyrethoid insecticides. The other is the conditional suicide plasmid for containment system, in which the lethal gene used was the nuclease gene of Serratia marcescens without the leader-coding sequence and was placed downstream of T7 promoter. The GEB has wide prospects of application on cleanup of pesticide residues with its degradability to several pesticides and containment system.

Expression and secretion of a lytic polysaccharide monooxygenase by a fast-growing cyanobacterium

BACKGROUND

Cyanobacteria have the potential to become next-generation cell factories due to their ability to use CO₂, light and inorganic nutrients to produce a range of biomolecules of commercial interest. Synechococcus elongatus UTEX 2973, in particular, is a fast-growing, genetically tractable, cyanobacterium that has garnered attention as a potential biotechnological chassis. To establish this unique strain as a host for heterologous protein production, we aimed to demonstrate expression and secretion of the industrially relevant TfAA10A, a lytic polysaccharide monooxygenase from the Gram-positive bacterium Thermobifida fusca.

RESULTS

Two variations of TfAA10A were successfully expressed in S. elongatus UTEX 2973: One containing the native N-terminal, Sec-targeted, signal peptide and a second with a Tat-targeted signal peptide from the Escherichia coli trimethylamine-N-oxide reductase (TorA). Although the TorA signal peptide correctly targeted the protein to the plasma membrane, the majority of the TorA-TfAA10A was found unprocessed in the plasma membrane with a small fraction of the mature protein ultimately translocated to the periplasm. The native Sec signal peptide allowed for efficient secretion of TfAA10A into the medium with virtually no protein being found in the cytosol, plasma membrane or periplasm. TfAA10A was demonstrated to be correctly cleaved and active on the model substrate phosphoric acid swollen cellulose. Additionally, expression and secretion only had a minor impact on cell growth. The secretion yield was estimated at 779 ± 40 µg L^-1 based on densitometric analysis. To our knowledge, this is the highest secretion yield ever registered in cyanobacteria.

CONCLUSIONS

We have shown for the first time high-titer expression and secretion of an industrially relevant and catalytically active enzyme in S. elongatus UTEX 2973. This proof-of-concept study will be valuable for the development of novel and sustainable applications in the fields of bioremediation and biocatalysis.

Surface Display of Small Affinity Proteins on Synechocystis sp. Strain PCC 6803 Mediated by Fusion to the Major Type IV Pilin PilA1

Functional surface display of small affinity proteins, namely, affibodies (6.5 kDa), was evaluated for the model cyanobacterium Synechocystis sp. strain PCC 6803 through anchoring to native surface structures. These structures included confirmed or putative subunits of the type IV pili, the S-layer protein, and the heterologous Escherichia coli autotransporter antigen 43 system. The most stable display system was determined to be through C-terminal fusion to PilA1, the major type IV pilus subunit in Synechocystis, in a strain unable to retract these pili (ΔpilT1). Type IV pilus synthesis was upheld, albeit reduced, when fusion proteins were incorporated. However, pilus-mediated functions, such as motility and transformational competency, were negatively affected. Display of affibodies on Synechocystis and the complementary anti-idiotypic affibodies on E. coli or Staphylococcus carnosus was able to mediate interspecies cell-cell binding by affibody complex formation. The same strategy, however, was not able to drive cell-cell binding and aggregation of Synechocystis-only mixtures. Successful affibody tagging of the putative minor pilin PilA4 showed that it locates to the type IV pili in Synechocystis and that its extracellular availability depends on PilA1. In addition, affibody tagging of the S-layer protein indicated that the domains responsible for the anchoring and secretion of this protein are located at the N and C termini, respectively. This study can serve as a basis for future surface display of proteins on Synechocystis for biotechnological applications.IMPORTANCE Cyanobacteria are gaining interest for their potential as autotrophic cell factories. Development of efficient surface display strategies could improve their suitability for large-scale applications by providing options for designed microbial consortia, cell immobilization, and biomass harvesting. Here, surface display of small affinity proteins was realized by fusing them to the major subunit of the native type IV pili in Synechocystis sp. strain PCC 6803. The display of complementary affinity proteins allowed specific cell-cell binding between Synechocystis and Escherichia coli or Staphylococcus carnosus Additionally, successful tagging of the putative pilin PilA4 helped determine its localization to the type IV pili. Analogous tagging of the S-layer protein shed light on the regions involved in its secretion and surface anchoring.

Cyanobacterial Surface Display System Mediates Engineered Interspecies and Abiotic Binding

Cyanobacteria are uniquely suited for the development of sustainable bioproduction platforms but are currently underutilized in scaled applications in part due to a lack of genetic tools. Here, we develop a surface display system in the cyanobacterial model Synechococcus elongatus PCC7942 via expression of modified versions of the outer membrane porin SomA. Importantly, we demonstrate accessibility of heterologous functional groups on the recombinant porin to the external environment in living cells. We show that this requires the removal of occluding factors that include lipopolysaccharides and a putative surface layer protein. Displayed epitopes on SomA can be utilized to mediate physical adhesion between living cyanobacteria and abiotic surfaces or an engineered Saccharomyces cerevisiae partner strain. We show that >80% of cyanobacterial cells attach to functionalized magnetic beads, allowing for magnet-assisted recovery. This work showcases the development of a functional surface display system in cyanobacteria with wide-ranging applications.

Peptide-mediated microalgae harvesting method for efficient biofuel production

BACKGROUND

Production of biofuels from microalgae has been recognized to be a promising route for a sustainable energy supply. However, the microalgae harvesting process is a bottleneck for industrialization because it is energy intensive. Thus, by displaying interactive protein factors on the cell wall, oleaginous microalgae can acquire the auto- and controllable-flocculation function, yielding smarter and energy-efficient harvesting.

RESULTS

Towards this goal, we established a cell-surface display system using the oleaginous diatom Fistulifera solaris JPCC DA0580. Putative cell wall proteins, termed frustulins, were identified from the genome information using a homology search. A selected frustulin was subsequently fused with green fluorescent protein (GFP) and a diatom cell-surface display was successfully demonstrated. The antibody-binding assay further confirmed that the displayed GFP could interact with the antibody at the outermost surface of the cells. Moreover, a cell harvesting experiment was carried out using silica-affinity peptide-displaying diatom cells and silica particles where engineered cells attached to the silica particles resulting in immediate sedimentation.

CONCLUSION

This is the first report to demonstrate the engineered peptide-mediated harvesting of oleaginous microalgae using a cell-surface display system. Flocculation efficiency based on the silica-affinity peptide-mediated cell harvesting method demonstrated a comparable performance to other flocculation strategies which use either harsh pH conditions or expensive chemical/biological flocculation agents. We propose that our peptide-mediated cell harvest method will be useful for the efficient biofuel production in the future.

The rrnA promoter as a tool for the improved expression of heterologous genes in cyanobacteria

The regulatory sequence of ribosomal RNA A (rrnA) operon from Synechococcus PCC7942 was characterized using green fluorescent protein gene (gfp) as a reporter. The PR promoter (nt. -83 to +2) including upstream promoter element and P1 promoter of rrnA exhibited GFP fluorescence intensity about 30-fold higher than full length sequence (nt. -147 to +79). The effects of PR promoter arranged in tandem with consensus-σ(70) promoter (PS) of Escherichia coli on the expression of gfp and opd gene encoding organophosphorus hydrolase (OPH) in Synechococcus were investigated. The PS-PR tandem promoter was superior to all of the other promoters; its GFP fluorescence intensity was a 1.8-fold increase when compared to PR-PR tandem promoter, a 2.5-fold, 9.5-fold and a 15-fold increase compared to PR, PS and promoter of tRNA(pro), respectively. The GFP from PS-PR tandem promoter accounted for about 12% of its total extracted proteins. OPH activity of Synechococcus harboring opd gene under the control of PS-PR tandem promoter was 738 ± 128 units/OD₇₃₀. We demonstrated that the tandem promoters remarkably enhanced the GFP and OPH production which were detected on SDS-PAGE stained with Coomassie blue. The promoter system in this study could be generally applied to production of valuable organic products from cyanobacteria.