Translocation of green fluorescent protein to cyanobacterial periplasm using ice nucleation protein

Surface display provides an efficient expression system for production of recombinant proteins and bacterial whole cell biosensor in E. coli

A novel bacterial display vector based on Escherichia coli has been engineered for recombinant protein production and purification. Accordingly, a construct harboring the enhanced green fluorescent protein (EGFP) and the ice nucleation protein (INP) was designed to produce EGFP via the surface display in E. coli cells. The fusion EGFP-expressed cells were then investigated using fluorescence measurement, SDS- and native-PAGE before and after TEV protease digestion. The displayed EGFP was obtained with a recovery of 57.7 % as a single band on SDS-PAGE. Next, the efficiency of the cell surface display for mutant EGFP (EGFP S202H/Q204H) was examined in sensing copper ions. Under optimal conditions, a satisfactorily linear range for copper ions concentrations up to 10 nM with a detection limit of 0.073 nM was obtained for cell-displayed mutant EGFP (mEGFP). In the presence of bacterial cell lysates and purified mEGFP, response to copper was linear in the 2-10 nM and 0.1-2 μM concentration range, respectively, with a 1.3 nM and 0.14 μM limit of detection. The sensitivity of bacterial cell lysates and surface-displayed mEGFP in the detection of copper ions is higher than the purified mEGFP.

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.

Chromosomal expression of CadR on Pseudomonas aeruginosa for the removal of Cd(II) from aqueous solutions

Genetically engineered bacteria for pollution control of heavy metal have been widely studied, however, using Pseudomonas aeruginosa (P. aeruginosa) that can adapt to various circumstances to remediate heavy metal pollution is rarely reported. In this study, we employed CadR, a cadmium (Cd)-specific binding protein, displaying on the surface of P. aeruginosa with chromosomal expression. The genetically engineered (GE) P. aeruginosa still flourished in the 30th generation in the LB broth which contained 100 μM Cd(II), exhibiting an excellent genetic stability. Chromosomally expressed P. aeruginosa showed an adsorption capacity of up to 131.9 μmol/g of Cd(II). In addition, the low concentration of the coexisting two valence ions has no significant effect on adsorption capacity of Cd(II). This study provides a direction for application of P. aeruginosa in environment remediation.

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.

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.

Arabinogalactan proteins occur in the free-living cyanobacterium genus Nostoc and in plant-Nostoc symbioses

Arabinogalactan proteins (AGP) are a diverse family of proteoglycans associated with the cell surfaces of plants. AGP have been implicated in a wide variety of plant cell processes, including signaling in symbioses. This study investigates the existence of putative AGP in free-living cyanobacterial cultures of the nitrogen-fixing, filamentous cyanobacteria Nostoc punctiforme and Nostoc sp. strain LBG1 and at the symbiotic interface in the symbioses between Nostoc spp. and two host plants, the angiosperm Gunnera manicata (in which the cyanobacterium is intracellular) and the liverwort Blasia pusilla (in which the cyanobacterium is extracellular). Enzyme-linked immunosorbent assay, immunoblotting, and immunofluorescence analyses demonstrated that three AGP glycan epitopes (recognized by monoclonal antibodies LM14, MAC207, and LM2) are present in free-living Nostoc cyanobacterial species. The same three AGP glycan epitopes are present at the Gunnera-Nostoc symbiotic interface and the LM2 epitope is detected during the establishment of the Blasia-Nostoc symbiosis. Bioinformatic analysis of the N. punctiforme genome identified five putative AGP core proteins that are representative of AGP classes found in plants. These results suggest a possible involvement of AGP in cyanobacterial-plant symbioses and are also suggestive of a cyanobacterial origin of AGP.

Molecular characterization of an ice nucleation protein variant (inaQ) from Pseudomonas syringae and the analysis of its transmembrane transport activity in Escherichia coli

The ice nucleation protein (INP) of Pseudomonas syringae has gained scientific interest not only because of its pathogenicity of foliar necroses but also for its wide range of potential applications, such as in snow making, frozen food preparation, and surface-display system development. However, studies on the transport activity of INP remain lacking. In the present study, a newly identified INP-gene variant, inaQ, from a P. syringae MB03 strain was cloned. Its structural domains, signal sequences, and the hydrophilicity or hydrophobicity of each domain, were then characterized. The deduced amino acid sequence of InaQ shares similar protein domains with three P. syringae INPs, namely, InaK, InaZ, and InaV, which were identified as an N-terminal domain, a central repeating domain, and a C-terminal domain. The expression of the full-length InaQ and of various truncated variants was induced in Escherichia coli to analyze their transmembrane transport and surface-binding activities, while using the green fluorescence protein (GFP) as the fusion partner. With two transmembrane segments and a weak secretion signal, the N-terminal domain (InaQ-N) alone was found to be responsible for the transport process as well as for the binding to the outer membrane, whereas the C-terminal region was nonfunctional in protein transport. Increased membrane transport and surface-binding capacities were induced by a low isopropyl-β-D-thiogalactoside concentration (0.1 mmol/l) but not by culture temperatures (15 ºC to 37 ºC). Furthermore, by constructing the GFP-fused proteins with a single InaQ-N, as well as two and three tandemly aligned InaQ-N molecules, the transport and membrane-binding activities of these proteins were compared using Western blot analysis, immmunofluorescence microscopy, and assays of the GFP specific fluorescence intensity of subcellular fractions and flow cytometry, which showed that the increase of InaQ-N repeats resulted in a coordinated increase of the surface-immobilization efficiency. Therefore, the results of this study can serve as a molecular basis for improving the performance of INP-based cell surface-display systems.

Translocation of green fluorescent protein by comparative analysis with multiple signal peptides

Type I and II secretory pathways are used for the translocation of recombinant proteins from the cytoplasm of Escherichia coli. The purpose of this study was to evaluate four signal peptides (HlyA, TorA, GeneIII, and PelB), representing the most common secretion pathways in E. coli, for their ability to target green fluorescent protein (GFP) for membrane translocation. Signal peptide-GFP genetic fusions were designed in accordance with BioFusion standards (BBF RFC 10, BBF RFC 23). The HlyA signal peptide targeted GFP for secretion to the extracellular media via the type I secretory pathway, whereas TAT-dependent signal peptide TorA and Sec-dependent signal peptide GeneIII exported GFP to the periplasm. The PelB signal peptide was inefficient in translocating GFP. The use of biological technical standards simplified the design and construction of functional signal peptide-recombinant protein genetic devices for type I and II secretion in E. coli. The utility of the standardized parts model is further illustrated as constructed biological parts are available for direct application to other studies on recombinant protein translocation.

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

The display of proteins to cyanobacterial cell surface is made complex by combination of Gram-positive and Gram-negative features of cyanobacterial cell wall. Here, we showed that Synechococcus outer membrane protein A (SomA) can be used as an anchoring motif for the display of organophosphorus hydrolase (OPH) on cyanobacterial cell surface. The OPH, capable of degrading a wide range of organophosphate pesticides, was fused in frame to the carboxyl-terminus of different cell-surface exposed loops of SomA. Proteinase K accessibility assay and immunostaining visualized under confocal laser scanning microscopy demonstrated that a minor fraction of OPH with 12 histidines fused in frame with the third cell-surface exposed loop of SomA (SomAL3-OPH12H) was displayed onto the outermost cell surface with a substantial fraction buried in the cell wall, whereas OPH fused in frame with the fifth cell-surface exposed loop of SomA (SomAL5-OPH) was successfully translocated across the membrane and completely displayed onto the outermost surface of Synechococcus. The successful display of the functional heterologous protein on cell surface provides a useful model for variety of applications in cyanobacteria including screening of polypeptide libraries and whole-cell biocatalysts by immobilizing enzymes.