Genetic Code Expansion in Shewanella oneidensis MR-1 Allows Site-Specific Incorporation of Bioorthogonal Functional Groups into a c-Type Cytochrome.

Genetic code expansion has enabled cellular synthesis of proteins containing unique chemical functional groups to allow the understanding and modulation of biological systems and engineer new biotechnology. Here, we report the development of efficient methods for site-specific incorporation of structurally diverse noncanonical amino acids (ncAAs) into proteins expressed in the electroactive bacterium Shewanella oneidensis MR-1. We demonstrate that the biosynthetic machinery for ncAA incorporation is compatible and orthogonal to the endogenous pathways of S. oneidensis MR-1 for protein synthesis, maturation of c-type cytochromes, and protein secretion. This allowed the efficient synthesis of a c-type cytochrome, MtrC, containing site-specifically incorporated ncAA in S. oneidensis MR-1 cells. We demonstrate that site-specific replacement of surface residues in MtrC with ncAAs does not influence its three-dimensional structure and redox properties. We also demonstrate that site-specifically incorporated bioorthogonal functional groups could be used for efficient site-selective labeling of MtrC with fluorophores. These synthetic biology developments pave the way to expand the chemical repertoire of designer proteins expressed in S. oneidensis MR-1.

Capacity and kinetics of light-induced cytochrome oxidation in intact cells of photosynthetic bacteria.

Light-induced oxidation of the reaction center dimer and periplasmic cytochromes was detected by fast kinetic difference absorption changes in intact cells of wild type and cytochrome mutants (cycA, cytC4 and pufC) of Rubrivivax gelatinosus and Rhodobacter sphaeroides. Constant illumination from a laser diode or trains of saturating flashes enabled the kinetic separation of acceptor and donor redox processes, and the electron contribution from the cyt bc1 complex via periplasmic cytochromes. Under continuous excitation, concentrations of oxidized cytochromes increased in three phases where light intensity, electron transfer rate and the number of reduced cytochromes were the rate liming steps, respectively. By choosing suitable flash timing, gradual steps of cytochrome oxidation in whole cells were observed; each successive flash resulted in a smaller, damped oxidation. We attribute this damping to lowered availability of reduced cytochromes resulting from both exchange (unbinding/binding) of the cytochromes and electron transfer at the reaction center interface since a similar effect is observed upon deletion of genes encoding periplasmic cytochromes. In addition, we present a simple model to calculate the damping effect; application of this method may contribute to understanding the function of the diverse range of c-type cytochromes in the electron transport chains of anaerobic phototrophic bacteria.

Enantioselective Synthesis of α-Trifluoromethyl Amines via Biocatalytic N-H Bond Insertion with Acceptor-Acceptor Carbene Donors.

The biocatalytic toolbox has recently been expanded to include enzyme-catalyzed carbene transfer reactions not occurring in Nature. Herein, we report the development of a biocatalytic strategy for the synthesis of enantioenriched α-trifluoromethyl amines through an asymmetric N-H carbene insertion reaction catalyzed by engineered variants of cytochrome c552 from Hydrogenobacter thermophilus. Using a combination of protein and substrate engineering, this metalloprotein scaffold was redesigned to enable the synthesis of chiral α-trifluoromethyl amino esters with up to >99% yield and 95:5 er using benzyl 2-diazotrifluoropropanoate as the carbene donor. When the diazo reagent was varied, the enantioselectivity of the enzyme could be inverted to produce the opposite enantiomers of these products with up to 99.5:0.5 er. This methodology is applicable to a broad range of aryl amine substrates, and it can be leveraged to obtain chemoenzymatic access to enantioenriched ß-trifluoromethyl-ß-amino alcohols and halides. Computational analyses provide insights into the interplay of protein- and reagent-mediated control on the enantioselectivity of this reaction. This work introduces the first example of a biocatalytic N-H carbenoid insertion with an acceptor-acceptor carbene donor, and it offers a biocatalytic solution for the enantioselective synthesis of α-trifluoromethylated amines as valuable synthons for medicinal chemistry and the synthesis of bioactive molecules.

Expression of Cytochrome c3 from Desulfovibrio vulgaris in Plant Leaves Enhances Uranium Uptake and Tolerance of Tobacco.

Cytochrome c3 (uranyl reductase) from Desulfovibrio vulgaris can reduce uranium in bacterial cells and in cell-free systems. This gene was introduced in tobacco under control of the RbcS promoter, and the resulting transgenic plants accumulated uranium when grown on a uranyl ion containing medium. The uptaken uranium was detected by EM in chloroplasts. In the presence of uranyl ions in sublethal concentration, the transgenic plants grew phenotypically normal while the control plants' development was impaired. The data on uranium oxidation state in the transgenic plants and the possible uses of uranium hyperaccumulation by plants for environmental cleanup are discussed.

C-type cytochrome-initiated reduction of bacterial lytic polysaccharide monooxygenases.

The release of glucose from lignocellulosic waste for subsequent fermentation into biofuels holds promise for securing humankind's future energy needs. The discovery of a set of copper-dependent enzymes known as lytic polysaccharide monooxygenases (LPMOs) has galvanised new research in this area. LPMOs act by oxidatively introducing chain breaks into cellulose and other polysaccharides, boosting the ability of cellulases to act on the substrate. Although several proteins have been implicated as electron sources in fungal LPMO biochemistry, no equivalent bacterial LPMO electron donors have been previously identified, although the proteins Cbp2D and E from Cellvibrio japonicus have been implicated as potential candidates. Here we analyse a small c-type cytochrome (CjX183) present in Cellvibrio japonicus Cbp2D, and show that it can initiate bacterial CuII/I LPMO reduction and also activate LPMO-catalyzed cellulose-degradation. In the absence of cellulose, CjX183-driven reduction of the LPMO results in less H2O2 production from O2, and correspondingly less oxidative damage to the enzyme than when ascorbate is used as the reducing agent. Significantly, using CjX183 as the activator maintained similar cellulase boosting levels relative to the use of an equivalent amount of ascorbate. Our results therefore add further evidence to the impact that the choice of electron source can have on LPMO action. Furthermore, the study of Cbp2D and other similar proteins may yet reveal new insight into the redox processes governing polysaccharide degradation in bacteria.

Identification and Validation of Four Novel Promoters for Gene Engineering with Broad Suitability across Species.

The transcriptional capacities of target genes are strongly influenced by promoters, whereas few studies have focused on the development of robust, high-performance and cross-species promoters for wide application in different bacteria. In this work, four novel promoters (Pk.rtufB, Pk.r1, Pk.r2, and Pk.r3) were predicted from Ketogulonicigenium robustum and their inconsistency in the -10 and -35 region nucleotide sequences indicated they were different promoters. Their activities were evaluated by using green fluorescent protein (gfp) as a reporter in different species of bacteria, including K. vulgare SPU B805, Pseudomonas putida KT2440, Paracoccus denitrificans PD1222, Bacillus licheniformis and Raoultella ornithinolytica, due to their importance in metabolic engineering. Our results showed that the four promoters had different activities, with Pk.r1 showing the strongest activity in almost all of the experimental bacteria. By comparison with the commonly used promoters of E. coli (tufB, lac, lacUV5), K. vulgare (Psdh, Psndh) and P. putida KT2440 (JE111411), the four promoters showed significant differences due to only 12.62% nucleotide similarities, and relatively higher ability in regulating target gene expression. Further validation experiments confirmed their ability in initiating the target minCD cassette because of the shape changes under the promoter regulation. The overexpression of sorbose dehydrogenase and cytochrome c551 by Pk.r1 and Pk.r2 resulted in a 22.75% enhancement of 2-KGA yield, indicating their potential for practical application in metabolic engineering. This study demonstrates an example of applying bioinformatics to find new biological components for gene operation and provides four novel promoters with broad suitability, which enriches the usable range of promoters to realize accurate regulation in different genetic backgrounds.

Refinement of the Griess method for measuring nitrite in biological samples.

Nitric oxide (NO) is a reactive gas that participates in many physiological as well as pathogenic processes in higher eukaryotic organisms. Inflammatory responses elicit higher levels of this molecule. Nevertheless, there are many technical challenges to accurately measure the amount of NO produced. Previously, a method using whole-cell extracts from Escherichia coli was able to generate the conversion of nitrate into nitrite to measure the amount of nitrate or indirectly the NO present in a sample using the Griess reaction. Here we present an improvement to this method, by using E. coli whole-cell extracts lacking one of the two nitrite reductases, rendered a more precise measurement when coupled with the Griess reaction than our previous report. Alternatively, osmotic stress showed to downregulate the expression of both nitrate reductases, which can be an alternative for indirect nitrate and NO reduction. The results presented here show an easy method for nitrate and NO reduction to nitrite and avoid the reconversion to nitrate, also as an alternative for other analytical methods that are based on cadmium, purified nitrate reductase enzyme, or salicylic methods to reduce NO. This method can be widely used for measuring NO production in living organisms, soil, and other relevant microbiological samples.

Enhancement of direct electron transfer in graphene bioelectrodes containing novel cytochrome c553 variants with optimized heme orientation.

The highly efficient bioelectrodes based on single layer graphene (SLG) functionalized with pyrene self-assembled monolayer and novel cytochromec553(cytc553)peptide linker variants were rationally designed to optimize the direct electron transfer (DET) between SLG and the heme group of cyt. Through a combination of photoelectrochemical and quantum mechanical (QM/MM) approaches we show that the specific amino acid sequence of a short peptide genetically inserted between the cytc553holoprotein and thesurface anchoring C-terminal His6-tag plays a crucial role in ensuring the optimal orientation and distance of the heme group with respect to the SLG surface. Consequently, efficient DET occurring between graphene and cyt c553 leads to a 20-fold enhancement of the cathodic photocurrent output compared to the previously reported devices of a similar type. The QM/MM modeling implies that a perpendicular or parallel orientation of the heme group with respect to the SLG surface is detrimental to DET, whereas the tilted orientation favors the cathodic photocurrent generation. Our work confirms the possibility of fine-tuning the electronic communication within complex bio-organic nanoarchitectures and interfaces due to optimization of the tilt angle of the heme group, its distance from the SLG surface and optimal HOMO/LUMO levels of the interacting redox centers.

Genome-scale mutant fitness reveals versatile c-type cytochromes in Shewanella oneidensis MR-1.

Shewanella has been widely investigated for its metabolic versatility and use of a large number of extracellular electron acceptors. Many c-type cytochromes are responsible for this diversity, mainly in condition-specific fashions. By using genome-scale mutant fitness data, we studied which genes (particularly c-type cytochromes) were used to coordinate various electron transfer processes in the present work. First, by integrating fitness profiles with protein-protein interaction (PPI) networks, we showed that the genes with a high total fitness value were generally more important in PPI networks than those with low fitness values. Then, we identified genes that are important across many experiments, and further fitness analysis confirmed five versatile c-type cytochromes: ScyA (SO0264), PetC (SO0610), CcoP (SO2361), CcoO (SO2363) and CytcB (SO4666), which are considered to be crucial in most experimental conditions. Finally, we demonstrated a mediating role in the periplasm for the less-reported CytcB by combining protein structure, subcellular localization and disordered region analysis. Comparative genome analysis further revealed that it is distinctive in Shewanella species. Collectively, these results suggest that periplasmic electron transfer processes are more diverse and flexible than previously reported, giving insight for further experimental studies of Shewanella oneidensis MR-1.

Heterologous production and functional characterization of Bradyrhizobium japonicum copper-containing nitrite reductase and its physiological redox partner cytochrome c550.

Two domain copper-nitrite reductases (NirK) contain two types of copper centers, one electron transfer (ET) center of type 1 (T1) and a catalytic site of type 2 (T2). NirK activity is pH-dependent, which has been suggested to be produced by structural modifications at high pH of some catalytically relevant residues. To characterize the pH-dependent kinetics of NirK and the relevance of T1 covalency in intraprotein ET, we studied the biochemical, electrochemical, and spectroscopic properties complemented with QM/MM calculations of Bradyrhizobium japonicum NirK (BjNirK) and of its electron donor cytochrome c550 (BjCycA). BjNirK presents absorption spectra determined mainly by a S(Cys)3pπ → Cu2+ ligand-to-metal charge-transfer (LMCT) transition. The enzyme shows low activity likely due to the higher flexibility of a protein loop associated with BjNirK/BjCycA interaction. Nitrite is reduced at high pH in a T1-decoupled way without T1 → T2 ET in which proton delivery for nitrite reduction at T2 is maintained. Our results are analyzed in comparison with previous results found by us in Sinorhizobium meliloti NirK, whose main UV-vis absorption features are determined by S(Cys)3pσ/π → Cu2+ LMCT transitions.

Rapid detection of cytochrome cd1-containing nitrite reductase encoding gene nirS of denitrifying bacteria with loop-mediated isothermal amplification assay.

The cytochrome cd1-containing nitrite reductase, nirS, plays an important role in biological denitrification. Consequently, investigating the presence and abundance of nirS is a commonly used approach to understand the distribution and potential activity of denitrifying bacteria, in addition to denitrifier communities. Herein, a rapid method for detecting nirS gene with loop-mediated isothermal amplification (LAMP) was developed, using Pseudomonas aeruginosa PAO1 (P. aeruginosa PAO1) as model microorganism to optimize the assay. The LAMP assay relied on a set of four primers that were designed to recognize six target sequence sites, resulting in high target specificity. The limit of detection for the LAMP assay under optimized conditions was 1.87 pg/reaction of genomic DNA, which was an order of magnitude lower than that required by conventional PCR assays. Moreover, it was validated that P. aeruginosa PAO1 cells as well as genomic DNA could be directly used as template. Only 1 h was needed from the addition of bacterial cells to the reaction to the verification of amplification success. The nirS gene of P. aeruginosa PAO1 in spiked seawater samples could be detected with both DNA-template based LAMP assay and cell-template based LAMP assay, demonstrating the practicality of in-field use.

Outer Membrane c-Type Cytochromes OmcA and MtrC Play Distinct Roles in Enhancing the Attachment of Shewanella oneidensis MR-1 Cells to Goethite.

The outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC in Shewanella are key terminal reductases that bind and transfer electrons directly to iron (hydr)oxides. Although the amounts of OmcA and MtrC at the cell surface and their molecular structures are largely comparable, MtrC is known to play a more important role in dissimilatory iron reduction. To explore the roles of these outer membrane c-Cyts in the interaction of Shewanella oneidensis MR-1 with iron oxides, the processes of attachment of S. oneidensis MR-1 wild type and c-type cytochrome-deficient mutants (the ΔomcA, ΔmtrC, and ΔomcA ΔmtrC mutants) to goethite are compared via quartz crystal microbalance with dissipation monitoring (QCM-D). Strains with OmcA exhibit a rapid initial attachment. The quantitative model for QCM-D responses reveals that MtrC enhances the contact area and contact elasticity of cells with goethite by more than one and two times, respectively. In situ attenuated total reflectance Fourier transform infrared two-dimensional correlation spectroscopic (ATR-FTIR 2D-CoS) analysis shows that MtrC promotes the initial interfacial reaction via an inner-sphere coordination. Atomic force microscopy (AFM) analysis demonstrates that OmcA enhances the attractive force between cells and goethite by about 60%. As a result, OmcA contributes to a higher attractive force with goethite and induces a rapid short-term attachment, while MtrC is more important in the longer-term interaction through an enhanced contact area, which promotes interfacial reactions. These results reveal that c-Cyts OmcA and MtrC adopt different mechanisms for enhancing the attachment of S. oneidensis MR-1 cells to goethite. It improves our understanding of the function of outer membrane c-Cyts and the influence of cell surface macromolecules in cell-mineral interactions.IMPORTANCEShewanella species are one group of versatile and widespread dissimilatory iron-reducing bacteria, which are capable of respiring insoluble iron minerals via six multiheme c-type cytochromes. Outer membrane c-type cytochromes (c-Cyts) OmcA and MtrC are the terminal reductases in this pathway and have comparable protein structures. In this study, we elucidate the different roles of OmcA and MtrC in the interaction of S. oneidensis MR-1 with goethite at the whole-cell level. OmcA confers enhanced affinity toward goethite and results in rapid attachment. Meanwhile, MtrC significantly increases the contact area of bacterial cells with goethite and promotes the interfacial reaction, which may explain its central role in extracellular electron transfer. This study provides novel insights into the role of bacterial surface macromolecules in the interfacial interaction of bacteria with minerals, which is critical to the development of a comprehensive understanding of cell-mineral interactions.

Tuning Extracellular Electron Transfer by Shewanella oneidensis Using Transcriptional Logic Gates.

Extracellular electron transfer (EET) pathways, such as those in the bacterium Shewanella oneidensis, interface cellular metabolism with a variety of redox-driven applications. However, designer control over EET flux in S. oneidensis has proven challenging because a functional understanding of its EET pathway proteins and their effect on engineering parametrizations (e.g., response curves, dynamic range) is generally lacking. To address this, we systematically altered transcription and translation of single genes encoding parts of the primary EET pathway of S. oneidensis, CymA/MtrCAB, and examined how expression differences affected model-fitted parameters for Fe(III) reduction kinetics. Using a suite of plasmid-based inducible circuits maintained by appropriate S. oneidensis knockout strains, we pinpointed construct/strain pairings that expressed cymA, mtrA, and mtrC with maximal dynamic range of Fe(III) reduction rate. These optimized EET gene constructs were employed to create Buffer and NOT gate architectures that predictably turn on and turn off EET flux, respectively, in response to isopropyl ß-D-1-thiogalactopyranoside (IPTG). Furthermore, we found that response functions generated by these logic gates (i.e., EET activity vs inducer concentration) were comparable to those generated by conventional synthetic biology circuits, where fluorescent reporters are the output. Our results provide insight on programming EET activity with transcriptional logic gates and suggest that previously developed transcriptional circuitry can be adapted to predictably control EET flux.

Evaluation of mtr cluster expression in Shewanella RCRI7 during uranium removal.

In recent years, bioremediation is considered as an efficient method to remove the pollutants from the industrial wastewater. In this study, quantitative gene expressions (Real-time RT-PCR) of mtr gene cluster (mtrA, mtrB, mtrC, mtrD, mtrE, mtrF and omcA) in five different uranium concentrations (0.1, 0.25, 0.5, 1 and 2 mM) were performed with ICP and microscopic live cell counting analysis under anaerobic condition, by Shewanella RCRI7 as a native bacterium. The results indicated that the amount of uranium removal and live-cell counting were decreased in the higher uranium concentrations (1 and 2 mM), due to the uranium toxicity, suggesting 0.5 mM as the optimum uranium concentration for Shewanella RCRI7 resistance. The expression of mtrCED and omcA genes presented increasing trend in the lower uranium concentrations (0.1, 0.25 and 0.5 mM) and a decreasing trend in 1 and 2 mM, while mtrABF, presented an inverse pattern, proving the alternative role of mtrF for mtrC and omcA, as the substantial multiheme cytochromes in Extracellular Electron Transfer (EET) pathway. These data are a proof of these gene vital roles in the EET pathway, proposing them for genetic engineering toward EET optimization, as the certain pathway in heavy metal bioremediation process.

Identification of an extracytoplasmic function sigma factor that facilitates c-type cytochrome maturation and current generation under electrolyte-flow conditions in Shewanella oneidensis MR-1.

Shewanella oneidensis MR-1 was cultured on electrodes in electrochemical flow cells (EFCs), and transcriptome profiles of electrode-attached cells grown under electrolyte-flow conditions were compared with those under static (nonflow) conditions. Results revealed that, along with genes related to c-type cytochrome maturation (e.g., dsbD), the SO_3096 gene encoding a putative extracytoplasmic function (ECF) sigma factor was significantly upregulated under electrolyte-flow conditions. Compared to wild-type MR-1 (WT), an SO_3096-deletion mutant (∆SO_3096) showed impaired biofilm formation and decreased current generation in EFCs, suggesting that SO_3096 plays critical roles in the interaction of MR-1 cells with electrodes under electrolyte-flow conditions. We also compared transcriptome profiles of WT and ∆SO_3096 grown in EFCs, confirming that many genes upregulated under the electrolyte-flow conditions, including dsbD, are regulated by SO_3096. LacZ reporter assays showed that transcription from a promoter upstream of dsbD is activated by SO_3096. Measurement of current generated by a dsbD-deletion mutant revealed that this gene is essential for the transfer of electrons to electrodes. These results indicate that the SO_3096 gene product facilitates c-type cytochrome maturation and current generation under electrolyte-flow conditions. The results also offer ecophysiological insights into how Shewanella regulates extracellular electron transfer to solid surfaces in the natural environment.

Geobacter cytochrome OmcZs binds riboflavin: implications for extracellular electron transfer.

Geobacter sulfurreducens is an important model organism for understanding extracellular electron transfer (EET), i.e. transfer of electrons from the cell's interior (quinone pool) to an extracellular substrate. This exoelectrogenic functionality can be exploited in bioelectrochemical applications. Nonetheless, key questions remain regarding the mechanisms of this functionality. G. sulfurreducens has been hypothesized to employ both multi-heme cytochromes and soluble, small molecule redox shuttles, as the final, redox-active species in EET. However, interactions between flavin redox shuttles and outer membrane, redox proteins in Geobacter have not been demonstrated. Herein, the heterologous expression and purification from E. coli of a soluble form of the multi-heme cytochrome OmcZs from G. sulfurreducens is reported. UV-vis absorption assays show that riboflavin can be reduced by OmcZs with concomitant oxidation of the protein. Fluorescence assays show that oxidized OmcZs and riboflavin interact with a binding constant of 34 µM. Furthermore, expression of OmcZs in E. coli enables EET in the host, and the current produced by these E. coli in a bioelectrochemical cell increases when riboflavin is introduced. These results support the hypothesis that OmcZs functions in EET by transiently binding riboflavin, which shuttles electrons from the outer membrane to the extracellular substrate.

Protein surface charge effect on 3D domain swapping in cells for c-type cytochromes.

Many c-type cytochromes (cyts) can form domain-swapped oligomers. The positively charged Hydrogenobacter thermophilus (HT) cytochrome (cyt) c552 forms domain-swapped oligomers during expression in the Escherichia coli (E. coli) expression system, but the factors influencing the oligomerization remain unrevealed. Here, we found that the dimer of the negatively charged Shewanella violacea (SV) cyt c5 exhibits a domain-swapped structure, in which the N-terminal helix is exchanged between protomers, similar to the structures of the HT cyt c552 and Pseudomonas aeruginosa (PA) cyt c551 domain-swapped dimers. Positively charged horse cyt c and HT cyt c552 domain swapped during expression in E. coli, whereas negatively charged PA cyt c551 and SV cyt c5 did not. Oligomers were formed during expression in E. coli for HT cyt c552 attached to either a co- or post-translational signal peptide for transportation through the cytoplasm membrane, but not for PA cyt c551 attached to either signal peptide. HT cyt c552 formed oligomers in E. coli in the presence and absence of rare codons. More oligomers were obtained from the in vitro folding of horse cyt c and HT cyt c552 by the addition of negatively charged liposomes during folding, whereas the amount of oligomers for the in vitro folding of PA cyt c551 and SV cyt c5 did not change significantly by the addition. These results indicate that the protein surface charge affects the oligomerization of c-type cyts in cells; positively charged c-type cyts assemble on a negatively charged membrane, inducing formation of domain-swapped oligomers during folding.

The electrifying physiology of Geobacter bacteria, 30 years on.

The family Geobacteraceae, with its only valid genus Geobacter, comprises deltaproteobacteria ubiquitous in soil, sediments, and subsurface environments where metal reduction is an active process. Research for almost three decades has provided novel insights into environmental processes and biogeochemical reactions not previously known to be carried out by microorganisms. At the heart of the environmental roles played by Geobacter bacteria is their ability to integrate redox pathways and regulatory checkpoints that maximize growth efficiency with electron donors derived from the decomposition of organic matter while respiring metal oxides, particularly the often abundant oxides of ferric iron. This metabolic specialization is complemented by versatile metabolic reactions, respiratory chains, and sensory networks that allow specific members to adaptively respond to environmental cues to integrate organic and inorganic contaminants in their oxidative and reductive metabolism, respectively. Thus, Geobacteraceae are important members of the microbial communities that degrade hydrocarbon contaminants under iron-reducing conditions and that contribute, directly or indirectly, to the reduction of radionuclides, toxic metals, and oxidized species of nitrogen. Their ability to produce conductive pili as nanowires for discharging respiratory electrons to solid-phase electron acceptors and radionuclides, or for wiring cells in current-harvesting biofilms highlights the unique physiological traits that make these organisms attractive biological platforms for bioremediation, bioenergy, and bioelectronics application. Here we review some of the most notable physiological features described in Geobacter species since the first model representatives were recovered in pure culture. We provide a historical account of the environmental research that has set the foundation for numerous physiological studies and the laboratory tools that had provided novel insights into the role of Geobacter in the functioning of microbial communities from pristine and contaminated environments. We pay particular attention to latest research, both basic and applied, that has served to expand the field into new directions and to advance interdisciplinary knowledge. The electrifying physiology of Geobacter, it seems, is alive and well 30 years on.

Improvement of the electron transfer rate in Shewanella oneidensis MR-1 using a tailored periplasmic protein composition.

Periplasmic c-type cytochromes are essential for the electron transport between the cytoplasmic membrane bound menquinol oxidase CymA and the terminal ferric iron reductase MtrABC in the outer membrane of Shewanella oneidensis cells. Either STC or FccA are necessary for periplasmic electron transfer. We followed the hypothesis that the elimination of potential competing reactions in the periplasm and the simultaneous overexpression of STC (cctA) could lead to an accelerated electron transfer to the cell surface. The genes nrfA, ccpA, napB and napA were replaced by cctA. This led to a 1.7-fold increased ferric iron reduction rate and a 23% higher current generation in a bioelectrochemical system. Moreover, the quadruple mutant had a higher periplasmic flavin content. Further deletion of fccA and its replacement by cctA resulted in a strain with ferric iron reduction rates similar to the wild type and a lower concentration of periplasmic flavin compared to the quadruple mutant. A transcriptomic analysis revealed that the quadruple mutant had a 3.7-fold higher cctA expression which could not be further increased by the replacement of fccA. This work indicates that a synthetic adaptation of Shewanella towards extracellular respiration holds potential for increased respiratory rates and consequently higher current densities.

Structural and Functional Relevance of the Conserved Residue V13 in the Triheme Cytochrome PpcA from Geobacter sulfurreducens.

The triheme cytochrome PpcA from Geobacter sulfurreducens is highly abundant under several growth conditions and is important for extracellular electron transfer. PpcA plays a central role in transferring electrons resulting from the cytoplasmic oxidation of carbon compounds to the cell exterior. This cytochrome is designed to couple electron and proton transfer at physiological pH, a process achieved via the selection of dominant microstates during the redox cycle of the protein, which are ultimately regulated by a well-established order of oxidation of the heme groups. The three hemes are covered only by a polypeptide chain of 71 residues and are located in the small hydrophobic core of the protein. In this work, we used NMR and X-ray crystallography to investigate the structural and functional role of a conserved valine residue (V13) located within van der Waals contact of hemes III and IV. The residue was replaced by alanine (V13A), isoleucine (V13I), serine (V13S), and threonine (V13T) to probe the effects of the side chain volume and polarity. All mutants were found to be as equally thermally stable as the native protein. The V13A and V13T mutants produced crystals and their structures were determined. The side chain of the threonine residue introduced in V13T showed two conformations, but otherwise the two structures did not show significant changes from the native structure. Analysis of the redox behavior of the four mutants showed that for the hydrophobic replacements (V13A and V13I) the redox properties, and hence the order of oxidation of the hemes, were unaffected in spite of the larger side chain, isoleucine, showing two conformations with minor changes of the protein in the heme core. On the other hand, the polar replacements (V13S and V13T) showed the presence of two more distinctive conformations, and the oxidation order of the hemes was altered. Overall, it is striking that a single residue with proper size and polarity, V13, was naturally selected to ensure a unique conformation of the protein and the order of oxidation of the hemes, endowing the cytochrome PpcA with the optimal functional properties necessary to ensure effectiveness in the extracellular electron transfer respiratory pathways of G. sulfurreducens.