Transparent, conductive, and ultrathin graphene films, as an alternative to the ubiquitously employed metal oxides window electrodes for solid-state dye-sensitized solar cells, are demonstrated. These graphene films are fabricated from exfoliated graphite oxide, followed by thermal reduction. The obtained films exhibit a high conductivity of 550 S/cm and a transparency of more than 70% over 1000-3000 nm. Furthermore, they show high chemical and thermal stabilities as well as an ultrasmooth surface with tunable wettability.

Amyloid formation is characterized by the conversion of soluble proteins into biochemically and structurally distinct fibers. Although amyloid formation is traditionally associated with diseases such as Alzheimer disease, a number of biologically functional amyloids have recently been described. Curli are amyloid fibers produced by Escherichia coli that contribute to biofilm formation and other important physiological processes. We characterized the polymerization properties of the major curli subunit protein CsgA. CsgA polymerizes into an amyloid fiber in a sigmoidal kinetic fashion with a distinct lag, growth, and stationary phase. Adding sonicated preformed CsgA fibers to the polymerization reaction can significantly shorten the duration of the lag phase. We also demonstrate that the conversion of soluble CsgA into an insoluble fiber involves the transient formation of an intermediate similar to that characterized for several disease-associated amyloids. The CsgA core amyloid domain can be divided into five repeating units that share sequence and structural hallmarks. We show that peptides representing three of these repeating units are amyloidogenic in vitro. Although the defining characteristics of CsgA polymerization appear conserved with disease-associated amyloids, these proteins evolved in diverse systems and for different purposes. Therefore, amyloidogenesis appears to be an innate protein folding pathway that can be capitalized on to fulfill normal physiological tasks.

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Amyloid fibers are filamentous proteinaceous structures commonly associated with mammalian neurodegenerative diseases. Nucleation is the rate-limiting step of amyloid propagation, and its nature remains poorly understood. Escherichia coli assembles functional amyloid fibers called curli on the cell surface using an evolved biogenesis machine. In vivo, amyloidogenesis of the major curli subunit protein, CsgA, is dependent on the minor curli subunit protein, CsgB. Here, we directly demonstrated that CsgB ؉ cells efficiently nucleated purified soluble CsgA into amyloid fibers on the cell surface. CsgA contains five imperfect repeating units that fulfill specific roles in directing amyloid formation. Deletion analysis revealed that the N-and C-terminal most repeating units were required for in vivo amyloid formation. We found that CsgA nucleation specificity is encoded by the N-and C-terminal most repeating units using a blend of genetic, biochemical, and electron microscopic analyses. In addition, we found that the C-terminal most repeat was most aggregation-prone and dramatically contributed to CsgA polymerization in vitro. This work defines the elegant molecular signatures of bacterial amyloid nucleation and polymerization, thereby revealing how nature directs amyloid formation to occur at the correct time and location.

Pretreatments such as dilute acid at elevated temperature are effective for the hydrolysis of pentose polymers in hemicellulose and also increase the access of enzymes to cellulose fibers. However, the fermentation of resulting syrups is hindered by minor reaction products such as furfural from pentose dehydration. To mitigate this problem, four genetic traits have been identified that increase furfural tolerance in ethanol-producing Escherichia coli LY180 (strain W derivative): increased expression of fucO, ucpA, or pntAB and deletion of yqhD. Plasmids and integrated strains were used to characterize epistatic interactions among traits and to identify the most effective combinations. Furfural resistance traits were subsequently integrated into the chromosome of LY180 to construct strain XW129 (LY180 ΔyqhD ackA::P yadC′ fucO-ucpA) for ethanol. This same combination of traits was also constructed in succinate biocatalysts (Escherichia coli strain C derivatives) and found to increase furfural tolerance. Strains engineered for resistance to furfural were also more resistant to the mixture of inhibitors in hemicellulose hydrolysates, confirming the importance of furfural as an inhibitory component. With resistant biocatalysts, product yields (ethanol and succinate) from hemicellulose syrups were equal to control fermentations in laboratory media without inhibitors. The combination of genetic traits identified for the production of ethanol (strain W derivative) and succinate (strain C derivative) may prove useful for other renewable chemicals from lignocellulosic sugars.lignocellulose | metabolic engineering

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Amyloid fibers are filamentous protein structures commonly associated with neurodegenerative diseases. Unlike disease-associated amyloids, which are the products of protein misfolding, Escherichia coli assemble membrane-anchored functional amyloid fibers called curli. Curli fibers are composed of two proteins, CsgA and CsgB. In vivo, the polymerization of the major curli subunit protein, CsgA, is dependent on CsgB-mediated nucleation. The amyloid core of CsgA features five imperfect repeats (R1-R5), and R1 and R5 govern CsgA responsiveness to CsgB nucleation and self-seeding by CsgA fibers. Here, the specificity of bacterial amyloid nucleation was probed, revealing that certain aspartic acid and glycine residues inhibit the intrinsic aggregation propensities and nucleation responsiveness of R2, R3, and R4. These residues function as "gatekeepers" to modulate CsgA polymerization efficiency and potential toxicity. A CsgA molecule lacking gatekeeper residues polymerized in vitro significantly faster than wild-type CsgA and polymerized in vivo in the absence of the nucleation machinery, resulting in mislocalized fibers. This uncontrolled polymerization was associated with cytotoxicity, suggesting that incorrectly regulated CsgA polymerization was detrimental to the cell.A myloids are ordered proteinaceous fibers commonly associated with mammalian neurodegenerative diseases and prion-based encephalopathies (1). Amyloid fibers have distinct biochemical and biophysical properties, such as remarkable resistance to chemical and thermal denaturation, and specific tinctorial properties when bound to Congo red and thioflavin T (ThT) (1). The molecular basis of neurodegenerative disease development induced by amyloid propagation remains elusive, partially because of the seemingly erratic and uncontrolled nature of amyloidogenesis. An emerging focus of amyloid biosynthesis has shown that amyloids can also be an integral part of physiology found in different organisms including bacteria, fungi, and mammals (2, 3). How nature coordinates functional amyloid propagation and reduces the associated cytotoxicity is poorly understood.Curli, a bacterially produced functional amyloid, is an important component of the extracellular matrix and is involved in bacterial community behaviors (4). Because of the amyloid properties of curli fibers (5, 6), the colonies of curli-producing Escherichia coli stain red when grown on Congo red indicator plates, which provides a convenient assay to monitor curli assembly in vivo (7). In E. coli, at least six proteins are dedicated to directing efficient curli formation. Curli fibers are composed of a major subunit CsgA and a minor subunit CsgB. CsgA remains unpolymerized until it encounters the surface-tethered nucleator CsgB, which initiates CsgA polymerization (8). CsgD is a transcriptional activator for the csgBA operon (4). CsgG, CsgE, and CsgF are nonfiber structural accessory proteins involved in secretion and stabilization of the fiber subunits and modulation of fiber assembly (6). CsgG is prop...

Furfural is an important fermentation inhibitor in hemicellulose sugar syrups derived from woody biomass. The metabolism of furfural by NADPH-dependent oxidoreductases, such as YqhD (low K m for NADPH), is proposed to inhibit the growth and fermentation of xylose in Escherichia coli by competing with biosynthesis for NADPH. The discovery that the NADH-dependent propanediol oxidoreductase (FucO) can reduce furfural provided a new approach to improve furfural tolerance. Strains that produced ethanol or lactate efficiently as primary products from xylose were developed. These strains included chromosomal mutations in yqhD expression that permitted the fermentation of xylose broths containing up to 10 mM furfural. Expression of fucO from plasmids was shown to increase furfural tolerance by 50% and to permit the fermentation of 15 mM furfural. Product yields with 15 mM furfural were equivalent to those of control strains without added furfural (85% to 90% of the theoretical maximum). These two defined genetic traits can be readily transferred to enteric biocatalysts designed to produce other products. A similar strategy that minimizes the depletion of NADPH pools by native detoxification enzymes may be generally useful for other inhibitory compounds in lignocellulosic sugar streams and with other organisms.