Recombinant Geobacter sp. Phosphatidylserine decarboxylase proenzyme (psd)

What is the biochemical function of phosphatidylserine decarboxylase?

Phosphatidylserine decarboxylase (PSD) catalyzes the decarboxylation of phosphatidylserine (PS) to form phosphatidylethanolamine (PE), representing a critical enzymatic reaction in phospholipid metabolism. This reaction occurs across diverse prokaryotic and eukaryotic species, highlighting the enzyme's evolutionary conservation. The conversion is particularly important for membrane biogenesis and function, as PE constitutes a major phospholipid component in bacterial and mitochondrial membranes. PSDs function as pyruvoyl-dependent enzymes, utilizing an unusual pyruvoyl prosthetic group within their active site to facilitate the decarboxylation reaction rather than employing a typical cofactor like pyridoxal phosphate . This distinctive catalytic mechanism distinguishes PSDs from many other decarboxylases and reflects their specialized evolutionary adaptation.

What structural characteristics define phosphatidylserine decarboxylase proenzymes?

Phosphatidylserine decarboxylases exist initially as inactive proenzymes that require post-translational processing to achieve catalytic activity. The defining structural characteristic of PSDs is their endoproteolytic cleavage, which generates two subunits: a larger β-subunit and a smaller α-subunit. This cleavage event creates the essential pyruvoyl prosthetic group at the N-terminus of the α-subunit, which serves as the catalytic center. Sequence analysis across multiple phyla reveals highly conserved motifs essential for this autoprocessing mechanism, including an FFXRX6RX12PXD motif containing a uniquely conserved aspartic acid, a PXXYHXXHXP motif with two conserved histidine residues, and a GS(S/T) motif with a conserved serine residue . These motifs place PSDs within the D-H-S serine protease family, with the conserved serine acting as the nucleophile in the autoprocessing reaction. The conserved GS(S/T) motif is particularly critical, as it represents the site of autocatalytic cleavage.

How is the proenzyme form of PSD processed into its active form?

The processing of phosphatidylserine decarboxylase from its proenzyme form to the active enzyme involves a precise autoendoproteolytic mechanism. Site-directed mutagenesis studies have demonstrated that this self-cleavage reaction depends critically on three conserved residues analogous to the catalytic triad in D-H-S serine proteases. Using Plasmodium knowlesi PSD (PkPSD) as a model, researchers identified Asp-139, His-198, and Ser-308 as essential for this process . The reaction begins with the serine residue within the GS(S/T) motif performing a nucleophilic attack, facilitated by the histidine and aspartate residues. This cleavage occurs in cis (within the same molecule rather than between molecules) and results in the formation of a pyruvoyl group at the newly created N-terminus of the α-subunit. Notably, the glycine residue (Gly-307) within the GS(S/T) motif is also essential for processing, while the third residue (Ser/Thr-309) is non-essential . This autoprocessing can be inhibited by phenylmethylsulfonyl fluoride (PMSF), confirming its mechanistic similarity to serine proteases.

What expression systems are most effective for recombinant PSD production?

For recombinant PSD production, multiple expression systems have proven effective, though each presents distinct advantages depending on research objectives. E. coli expression systems offer simplicity and high yield for basic biochemical studies, particularly for bacterial PSDs. When expressing recombinant Geobacter sp. PSD, codon optimization is often necessary due to potential codon usage bias. This approach was successfully demonstrated with Plasmodium falciparum PSD, where increasing the G+C content from 28.7% to 35.03% and optimizing codon usage frequency from 0.70 to 0.83 significantly improved expression . For eukaryotic PSDs, including those being studied as models for Geobacter sp. enzyme characterization, yeast expression systems provide a eukaryotic cellular environment that can facilitate proper folding and processing. Baculovirus expression systems offer advantages for larger-scale production of more complex PSDs requiring specific post-translational modifications . When selecting an expression system, researchers should consider the subcellular localization of the native enzyme—many PSDs are membrane-associated, necessitating strategies to address solubility and proper folding of the recombinant protein.

What purification strategies maximize recovery of properly processed PSD?

Purification of recombinant phosphatidylserine decarboxylase presents unique challenges due to its autoprocessing requirement and potential membrane association. An effective purification strategy begins with careful lysis conditions that maintain the native conformation while efficiently extracting the protein. For Geobacter sp. PSD purification, affinity chromatography using N-terminal or C-terminal tags represents the primary approach, with important considerations for tag positioning to avoid interference with processing . Since improper tag placement could hinder the autoendoproteolytic cleavage essential for PSD activation, researchers should verify proper processing through SDS-PAGE analysis, confirming the presence of both α and β subunits. Purification to ≥85% homogeneity is typically achievable and sufficient for most biochemical studies . For structural studies requiring higher purity, additional chromatographic steps such as ion exchange or size exclusion are recommended. When working with the proenzyme form, purification should be conducted rapidly at lower temperatures (4°C) to minimize premature processing, and the inclusion of protease inhibitors (excluding PMSF, which would inhibit the desired autoprocessing) can protect against degradation by contaminating proteases.

How can researchers confirm proper autoprocessing of recombinant PSD?

Verification of proper autoprocessing is critical for ensuring that recombinant phosphatidylserine decarboxylase has achieved its functional form. The most direct method is SDS-PAGE analysis, which should reveal the presence of both α and β subunits with apparent molecular weights corresponding to the cleaved products rather than the full-length proenzyme. When expressing tagged constructs, western blotting can provide additional confirmation by detecting either the N-terminal or C-terminal fragments depending on tag placement. Researchers working with Geobacter sp. PSD should expect to observe the characteristic processing pattern similar to other bacterial PSDs. Mass spectrometry provides a more precise verification method, capable of confirming the exact cleavage site and detecting the formation of the pyruvoyl group at the N-terminus of the α-subunit. Functional validation through enzymatic activity assays serves as the ultimate confirmation of proper processing, as only correctly processed enzyme will display catalytic activity . A complementary approach involves heterologous expression in PSD-deficient yeast strains (such as psd1Δpsd2Δdpl1Δ), where successful complementation of the ethanolamine auxotrophy provides strong evidence of proper processing and activity .

What assays are most reliable for measuring PSD enzymatic activity?

For reliable measurement of PSD enzymatic activity, researchers have several methodological options depending on their specific experimental requirements. The most direct approach utilizes radiolabeled substrates, particularly [³H]-serine-labeled phosphatidylserine, to monitor the conversion to phosphatidylethanolamine. This method provides excellent sensitivity and allows for quantification of reaction products through thin-layer chromatography (TLC) and scintillation counting . For researchers working with Geobacter sp. PSD, this approach would enable precise determination of enzymatic parameters. An alternative non-radioactive method involves coupling the decarboxylation reaction to the detection of released CO₂, either through pH-sensitive indicators or specialized CO₂ detection systems. Mass spectrometry-based assays offer another powerful approach, enabling direct quantification of substrate depletion and product formation without radiolabeling. When comparing activity levels across enzyme preparations, it's essential to normalize activity to protein concentration and to confirm that the enzyme preparation has undergone proper autoprocessing. The (PE+PC)/PS ratio in cellular phospholipid composition serves as a useful indicator of in vivo PSD activity, with values between 2.0-3.8 typically observed in systems with functional PSDs .

How can substrate specificity of PSD be systematically evaluated?

Systematic evaluation of PSD substrate specificity requires a structured approach examining various phospholipid substrates and their structural analogs. For Geobacter sp. PSD characterization, researchers should first establish baseline activity with natural phosphatidylserine species varying in fatty acid composition, as substrate preference may exist for specific acyl chain lengths or saturation levels. A comprehensive substrate panel should include:

Michaelis-Menten kinetic analysis should be performed for preferred substrates to determine Km and Vmax values. Additionally, researchers should investigate whether the enzyme exhibits any secondary activities, such as direct serine decarboxylation—a property that has been evaluated in other PSDs and typically found to be absent . Complementation studies in yeast mutants lacking specific phospholipid biosynthetic capabilities provide powerful tools for in vivo assessment of substrate utilization, as demonstrated with Plasmodium falciparum PSD expressed in the yeast pss1 mutant to evaluate potential serine decarboxylase activity .

What approaches can determine the subcellular localization of PSD in Geobacter species?

Determining the subcellular localization of phosphatidylserine decarboxylase in Geobacter species requires multiple complementary approaches to provide confident assignment. Computational prediction represents the initial step, utilizing algorithms that analyze the protein sequence for targeting signals, transmembrane domains, and sorting motifs. For experimental verification in native Geobacter, cellular fractionation followed by western blotting or enzymatic activity assays can identify which cellular compartment contains PSD activity. Immunofluorescence or immunoelectron microscopy using antibodies against the native enzyme provides higher resolution localization, though this requires developing specific antibodies against Geobacter PSD. For recombinant studies, fluorescent protein fusions can track localization in live cells, with careful consideration of whether N- or C-terminal tagging might disrupt localization signals or processing. Heterologous expression systems can provide additional insights—for instance, complementation studies in yeast mutants have indicated that Plasmodium falciparum PSD localizes to the endoplasmic reticulum , while the human ortholog PISD targets to the inner mitochondrial membrane via an N-terminal targeting sequence . By combining these approaches, researchers can establish whether Geobacter PSD associates with the cytoplasmic membrane, specific cellular compartments, or potentially multiple locations depending on cellular conditions.

How do structural elements of PSD contribute to its autocatalytic processing mechanism?

The autocatalytic processing mechanism of phosphatidylserine decarboxylase represents a fascinating example of molecular evolution where an enzyme begins its existence as a protease before transforming into a decarboxylase. Structural elements critical to this mechanism have been identified through sequence conservation analysis and site-directed mutagenesis studies. The mechanism centers around three key conserved residues that function analogously to a serine protease catalytic triad: an aspartic acid within an FFXRX6RX12PXD motif, two histidines within a PXXYHXXHXP motif, and a serine within a GS(S/T) motif . In Plasmodium knowlesi PSD, these correspond to Asp-139, His-198, and Ser-308, respectively. The glycine preceding the serine (Gly-307) is also essential, likely providing the conformational flexibility needed for proper positioning of the serine nucleophile. Structural studies of PSDs from various organisms suggest that these residues come into proximity in the folded proenzyme, creating the catalytic center needed for the self-cleavage reaction. Deletion studies with PfPSD identified structural elements between positions 60-70 that are necessary for proteolytic processing—deletions beyond this point prevented proenzyme cleavage . This region may facilitate proper folding of the catalytic domain or contribute essential residues to the processing mechanism. Understanding these structural elements provides insights into the evolutionary adaptation of this unique enzyme family and offers potential targets for selective inhibitor design.

What is known about the transcriptional regulation of psd genes across different bacterial species?

Transcriptional regulation of phosphatidylserine decarboxylase genes varies across bacterial species, reflecting their diverse metabolic roles and environmental adaptations. In Escherichia coli, detailed studies have revealed that the psd gene is part of an operon (psd-mscM) subject to dual regulation by two major stress response systems: the σE (sigma E) extracytoplasmic stress response and the CpxR envelope stress response . This dual regulation was demonstrated through GFP transcriptional fusion experiments, which identified two distinct promoters: psdPσE, which is strongly induced by σE overproduction, and psdP2, which is positively regulated by CpxR even under balanced growth conditions . Mutations in the predicted -10 box of the psdPσE promoter abolished induction by σE, confirming its role in stress-responsive regulation. While less is known specifically about Geobacter sp. PSD regulation, its expression likely responds to membrane stress conditions similar to other bacteria. The integration of PSD expression into stress response networks makes biological sense, as perturbations to membrane integrity would require adjusted phospholipid biosynthesis. Comparative genomic approaches examining the upstream regions of psd genes across diverse bacterial species, including Geobacter, could reveal conserved regulatory elements and transcription factor binding sites. Researchers investigating Geobacter sp. PSD regulation should consider examining expression under various stress conditions that might affect membrane composition and integrity.

How can researchers develop specific inhibitors targeting phosphatidylserine decarboxylase?

Developing specific inhibitors targeting phosphatidylserine decarboxylase requires a systematic approach based on understanding the unique aspects of its structure and mechanism. Despite the validation of this enzyme class as a suitable target for antimicrobial development, no specific inhibitors have been reported in the literature . For Geobacter sp. PSD, inhibitor development strategies should focus on several potential approaches. Structure-based design, utilizing molecular modeling of the active site containing the pyruvoyl prosthetic group, can guide the creation of transition state analogs that specifically interact with this unusual catalytic center. Screening chemical libraries against purified, activated enzyme provides an empirical approach to identify lead compounds. Researchers should also consider designing mechanism-based inhibitors that target the unique autoprocessing reaction, potentially preventing the formation of the active enzyme. This approach has proven successful with other self-cleaving enzymes. Alternatively, non-hydrolyzable phosphatidylserine analogs could serve as competitive inhibitors. When evaluating potential inhibitors, it's essential to test for specificity against related pyruvoyl-dependent enzymes and to assess their effects in cellular systems through complementation studies in PSD-deficient yeast strains. Phenylmethylsulfonyl fluoride (PMSF) has been shown to inhibit the post-translational processing of PSD , suggesting that modified sulfonyl fluorides might serve as a starting point for developing processing-specific inhibitors.

What strategies address poor expression or incomplete processing of recombinant PSD?

Poor expression or incomplete processing of recombinant phosphatidylserine decarboxylase represents a common challenge that requires systematic troubleshooting. For Geobacter sp. PSD, codon optimization should be the first consideration, as the high A+T content typical of some bacterial genomes can impair expression in heterologous systems. Studies with Plasmodium falciparum PSD demonstrated significant improvement after increasing G+C content and optimizing codon usage frequency . Expression temperature represents another critical parameter—lower temperatures (16-25°C) often enhance proper folding of complex proteins at the expense of expression rate. If membrane association impairs solubility, researchers should consider expressing only the catalytic domain or utilizing fusion partners that enhance solubility. For addressing incomplete processing, researchers should verify that the construct includes all regions necessary for autoprocessing. Studies with PfPSD identified structural elements between amino acids 60-70 as essential for processing—truncated constructs lacking this region failed to undergo proper cleavage . Importantly, mixing experiments demonstrated that processing occurs primarily in cis rather than trans, indicating that each protein molecule must fold properly to achieve self-cleavage . For challenging Geobacter constructs, expression in PSD-deficient yeast strains offers a eukaryotic environment that may better support proper folding while providing a functional complementation assay to confirm activity.

How can researchers distinguish between defects in PSD processing versus catalytic activity?

Distinguishing between defects in processing versus catalytic activity is essential for accurately characterizing phosphatidylserine decarboxylase variants or when troubleshooting expression issues. This distinction requires a methodical approach combining structural and functional analyses. Initially, SDS-PAGE analysis can determine whether the proenzyme has undergone cleavage into α and β subunits, which is prerequisite for activity. Mass spectrometry provides more definitive evidence of processing, confirming both the cleavage event and formation of the pyruvoyl prosthetic group at the newly created N-terminus. For variants that show processing but lack activity, the defect lies in the catalytic machinery rather than in autoprocessing. Complementation studies in PSD-deficient yeast strains provide a powerful functional assessment—failure to rescue growth in the absence of ethanolamine despite evidence of processing indicates a catalytic defect . Researchers working with Geobacter sp. PSD should consider creating targeted mutations in the conserved catalytic residues to establish structure-function relationships. The ratio of downstream phospholipid products (PE+PDME+PC) to PS in cellular lipid profiles serves as an excellent indicator of in vivo activity, with properly functioning PSDs typically showing ratios approximately double those of strains lacking PSD activity . This comprehensive approach enables researchers to precisely locate the defect in the enzyme's functional pathway, distinguishing processing issues from catalytic deficiencies.

What approaches help overcome challenges in structural studies of membrane-associated PSDs?

Structural studies of membrane-associated phosphatidylserine decarboxylases present significant challenges due to their amphipathic nature and processing requirements. For researchers tackling structural analysis of Geobacter sp. PSD, several approaches can enhance success probability. Protein engineering represents a primary strategy—creating truncated constructs that retain the catalytic domain while removing transmembrane or membrane-association regions can improve solubility and crystallization properties. When designing these constructs, researchers should preserve regions essential for autoprocessing, particularly the identified structural elements between positions 60-70 that are necessary for proteolytic cleavage . Co-expression with binding partners or use of fusion tags specifically designed for membrane protein crystallization can stabilize the protein structure. For X-ray crystallography approaches, detergent screening is crucial for extracting and maintaining protein stability—a panel of mild detergents (DDM, LMNG, CHAPS) should be systematically evaluated. Cryo-electron microscopy offers advantages for membrane proteins without requiring crystallization, particularly suitable for larger complexes. Lipid cubic phase crystallization provides an alternative approach specifically developed for membrane proteins. For low-resolution structural insights, small-angle X-ray scattering (SAXS) with detergent-solubilized protein can provide envelope information about protein dimensions and quaternary structure. Ultimately, successful structural studies may require combining multiple approaches, such as crystallizing the soluble catalytic domain while using computational methods to model membrane-association regions.

How might evolutionary analysis of PSD enzymes reveal functional adaptations across species?

Evolutionary analysis of phosphatidylserine decarboxylase enzymes across diverse species offers profound insights into functional adaptations related to membrane biology and metabolism. For researchers interested in Geobacter sp. PSD, comparative genomic approaches can identify lineage-specific adaptations within this genus and related δ-proteobacteria. Phylogenetic reconstruction using PSD sequences from diverse prokaryotes and eukaryotes would illuminate the evolutionary trajectory of this enzyme family and potentially reveal horizontal gene transfer events that contributed to its distribution. The uniquely conserved motifs—FFXRX6RX12PXD, PXXYHXXHXP, and GS(S/T)—serve as evolutionary markers that can be traced across lineages to understand how the critical autoprocessing mechanism has been preserved . Coevolutionary analysis comparing PSD sequences with their membrane environments (analyzed through lipidomic approaches) could reveal adaptations to specific membrane compositions or environmental niches. Particularly interesting would be investigating whether PSDs from extremophiles (thermophiles, halophiles, psychrophiles) show structural adaptations corresponding to their environmental challenges. Positive selection analysis can identify rapidly evolving residues that might contribute to specialized functions in particular lineages, potentially revealing whether Geobacter sp. PSD has unique adaptations related to its role in extracellular electron transfer processes characteristic of this genus.

What potential roles might PSD inhibitors play in antimicrobial development?

Phosphatidylserine decarboxylase inhibitors represent promising candidates for antimicrobial development due to the enzyme's critical role in bacterial phospholipid metabolism. Genetic studies have validated the phosphatidylethanolamine synthesis pathway from phosphatidylserine as a suitable target, though no inhibitors of this enzyme class have been reported . For developing antimicrobials targeting Geobacter sp. or other bacteria, PSD inhibitors offer several advantages. The unique processing mechanism and pyruvoyl-dependent catalytic activity present opportunities for high specificity compared to more common enzyme families. Structure-based design approaches targeting either the autoprocessing mechanism or the mature enzyme active site could yield highly selective inhibitors. Particularly promising would be compounds that exploit structural differences between bacterial and mammalian PSDs to achieve selective toxicity. The essentiality of phosphatidylethanolamine in many bacterial membranes suggests that effective PSD inhibitors would have bactericidal rather than bacteriostatic effects. For Geobacter specifically, disrupting PE synthesis might have additional impacts on their distinctive extracellular electron transfer capabilities, potentially interfering with processes like metal reduction or electrode interactions important in bioelectrochemical applications. Researchers developing PSD inhibitors should utilize phenylmethylsulfonyl fluoride's known inhibition of PSD processing as a structural starting point, while also exploring transition state analogs that target the pyruvoyl-dependent decarboxylation reaction.

How can advanced protein engineering approaches improve recombinant PSD for biotechnological applications?

Advanced protein engineering approaches offer significant potential for enhancing recombinant phosphatidylserine decarboxylase for various biotechnological applications. For Geobacter sp. PSD optimization, rational design based on structural understanding represents a powerful approach. Engineering the enzyme's N-terminal region could improve processing efficiency, particularly focusing on the structural elements between positions 60-70 that have been identified as essential for proteolytic cleavage . Stability engineering through the introduction of disulfide bridges or consensus-based design could enhance the enzyme's thermal stability and solvent tolerance, expanding its utility in biocatalytic applications. Domain fusion approaches, linking PSD with other enzymes involved in phospholipid synthesis, could create efficient multi-enzyme complexes for in vitro phospholipid production systems. For industrial applications requiring large-scale production, directed evolution strategies targeting improved expression, solubility, and catalytic efficiency could yield variants with enhanced properties. Compartmentalization approaches, such as incorporating the enzyme into nanodiscs or liposomes, might improve activity with membrane-bound substrates while enhancing stability. Surface display technologies could enable the development of whole-cell biocatalysts with accessible PSD activity for phospholipid modification applications. These engineering approaches could ultimately lead to optimized PSD variants suitable for applications in lipidomics, membrane engineering, and pharmaceutical phospholipid production.

How can researchers combine genetic, biochemical, and structural approaches to fully characterize PSD function?

Comprehensive characterization of phosphatidylserine decarboxylase function requires integrating genetic, biochemical, and structural methodologies into a cohesive research program. For Geobacter sp. PSD investigation, genetic approaches should include gene deletion studies to establish essentiality and physiological roles, complementation analyses using heterologous expression in model organisms like PSD-deficient yeast strains, and site-directed mutagenesis targeting conserved residues to establish structure-function relationships. Biochemical characterization should encompass activity assays using radiolabeled substrates or mass spectrometry, detailed kinetic analyses with various substrates and potential inhibitors, and processing studies to understand the autoendoproteolytic mechanism. Structural biology approaches, including X-ray crystallography of soluble domains and cryo-electron microscopy for membrane-associated regions, can provide atomic-level insights into the enzyme's architecture. Lipidomic analyses comparing wild-type and PSD-deficient strains reveal the enzyme's impact on cellular phospholipid composition, particularly quantifying PE/PS ratios as indicators of in vivo activity . Systems biology approaches, including transcriptomics and metabolomics, can identify compensatory pathways activated upon PSD disruption. Computational modeling integrating these diverse data types can generate testable hypotheses about regulatory mechanisms and catalytic processes. This multi-faceted approach enables researchers to connect molecular mechanisms to cellular functions, providing a comprehensive understanding of PSD's role in Geobacter physiology and potential biotechnological applications.

What are the implications of PSD regulation for bacterial stress responses and adaptation?

The regulation of phosphatidylserine decarboxylase has significant implications for bacterial stress responses and adaptation, particularly through its effects on membrane composition and integrity. In Escherichia coli, the dual regulation of the psd gene by both the σE extracytoplasmic stress response and the CpxR envelope stress response systems indicates its central role in maintaining membrane homeostasis during environmental challenges. For Geobacter species, which encounter diverse environmental stresses in their natural habitats, PSD regulation likely represents a critical adaptive mechanism. Changes in phosphatidylethanolamine content directly impact membrane fluidity, permeability, and protein function—properties that must be modulated in response to temperature fluctuations, pH changes, osmotic stress, and exposure to toxic compounds. The integration of PSD expression into stress response networks enables bacteria to rapidly adjust membrane composition, potentially conferring resistance to antimicrobial compounds that target membrane integrity. During adaptation to new environments, altered PSD expression or activity could facilitate long-term changes in membrane composition that optimize cellular function under specific conditions. For Geobacter species in particular, which utilize extracellular electron transfer mechanisms requiring specialized membrane proteins, PSD regulation might influence their capacity for metal reduction or electricity generation in microbial fuel cells. Understanding these regulatory mechanisms could provide insights into bacterial adaptation strategies and potentially reveal approaches to modulate bacterial behavior in environmental or industrial applications.

How can synthetic biology approaches leverage PSD function for membrane engineering applications?

Synthetic biology approaches offer exciting possibilities for leveraging phosphatidylserine decarboxylase function in membrane engineering applications. For researchers working with Geobacter sp. PSD, several innovative strategies warrant exploration. Controlled expression systems could enable precise modulation of cellular phosphatidylethanolamine levels, creating designer membranes with specific fluidity, permeability, and curvature properties. Substrate engineering approaches, combining PSD with engineered phosphatidylserine synthases, could generate novel phospholipid compositions incorporating non-standard headgroups or fatty acids. Subcellular targeting through fusion with localization signals could direct PSD activity to specific membrane compartments, enabling spatially-resolved membrane modification. Cell-free phospholipid synthesis platforms incorporating purified PSD alongside other phospholipid biosynthetic enzymes could produce tailored liposomes for drug delivery, artificial cell systems, or biocatalytic applications. For Geobacter species specifically, engineered PSDs could potentially enhance their extracellular electron transfer capabilities by optimizing membrane composition for cytochrome incorporation or conductivity. Multi-enzyme scaffolding approaches, co-localizing PSD with other phospholipid biosynthetic enzymes, could create efficient metabolic channels for directed membrane synthesis. These synthetic biology applications could ultimately lead to engineered cells with enhanced stress tolerance, improved production of membrane-associated products, or novel capabilities for environmental applications like bioremediation or bioelectricity generation.