ABSTRACT
A preheated high-temperature environment is believed to be critical for a chemical-exfoliation-based production of graphenes starting from graphite oxide, a belief that is based on not only experimental but also theoretical viewpoints. A novel exfoliation approach is reported in this study, and the exfoliation process is realized at a very low temperature, which is far below the proposed critical exfoliation temperature, by introducing a high vacuum to the exfoliation process. Owing to unique surface chemistry, low-temperature exfoliated graphenes demonstrate an excellent energy storage performance, and the electrochemical capacitance is much higher than that of the high-temperature exfoliated ones. The low-temperature exfoliation approach presents us with a possibility for a mass production of graphenes at low cost and great potentials in energy storage applications of graphene-based materials.
ABSTRACT
A binding site for the Escherichia coli nucleoid binding protein FIS (factor for inversion stimulation) was identified upstream of a sigma54-dependent promoter, glnAp2. The binding and bending center of FIS is positioned at -55 with respect to the transcription start site (+1). Binding of FIS at this site activates the transcription of glnAp2 both in vivo and in vitro. Furthermore, we substituted the FIS-mediated DNA bending with other protein (cAMP receptor protein or integration host factor)-mediated DNA bending, without changing the position of the bending center. In vitro transcription assays indicated that all DNA bends centered at -55 activate transcriptional initiation of glnAp2, especially when linear templates were used.
Subject(s)
DNA, Bacterial/metabolism , Escherichia coli/metabolism , Factor For Inversion Stimulation Protein/metabolism , Gene Expression Regulation, Bacterial , Glutamate-Ammonia Ligase/metabolism , Base Sequence , DNA, Bacterial/chemistry , DNA, Bacterial/genetics , Enhancer Elements, Genetic , Escherichia coli/genetics , Escherichia coli Proteins/genetics , Escherichia coli Proteins/metabolism , Factor For Inversion Stimulation Protein/genetics , Glutamate-Ammonia Ligase/chemistry , Glutamate-Ammonia Ligase/genetics , Molecular Sequence Data , Nucleic Acid Conformation , Promoter Regions, Genetic , Transcription, GeneticABSTRACT
Sigma54-RNA polymerase (Esigma54) predominantly contacts one face of the DNA helix in the closed promoter complex, and interacts with the upstream enhancer-bound activator via DNA looping. Up to date, the precise face of Esigma54 that contacts the activator to convert the closed complex to an open one remains unclear. By introducing protein-induced DNA bends at precise locations between upstream enhancer sequences and the core promoter of the sigma54-dependent glnAp2 promoter without changing the distance in-between, we observed a strong enhanced or decreased promoter activity, especially on linear DNA templates in vitro. The relative positioning and orientations of Esigma54, DNA bending protein and enhancer-bound activator on linear DNA were determined by in vitro footprinting analysis. Intriguingly, the locations from which the DNA bending protein exerted its optimal stimulatory effects were all found on the opposite face of the DNA helix compared with the DNA bound Esigma54 in the closed complex. Therefore, these results provide evidence that the activator must approach the Esigma54 closed complexes from the unbound face of the promoter DNA helix to catalyse open complex formation. This proposal is further supported by the modelling of activator-promoter DNA-Esigma54 complex.