ABSTRACT
The roles of bulk surface states and interfacial defects are probed experimentally using a combination of current-voltage, capacitance-voltage, and impedance measurements. The critical importance of the quality of both the film and interfaces is evident in current-voltage measurements where shunting and interface states result in large dark currents and the subsequent loss of Jsc. These properties are shown to be critically related to the nature and role of the PbS QD interface with the (nominally) ohmic gold contact. Specifically, the nonideality of this interface results in the formation of an electric field and therefore a Schottky barrier that opposes the transport of carriers across the conventional ZnO-PbS CQD system. Nonidealities in the structure and absorber layer are also reflected in nonmonotonic behavior and dispersion in C-V measurements with trapping processes on the CQD surfaces, and the ZnO/PbS and PbS/Au interfaces also affecting the carrier dynamics, which is reflected in the response time of these systems under different biases.
ABSTRACT
NmrA is a negative transcriptional regulator involved in the post-translational modulation of the GATA-type transcription factor AreA, forming part of a system controlling nitrogen metabolite repression in various fungi. X-ray structures of two NmrA crystal forms, both to 1.8 A resolution, show NmrA consists of two domains, including a Rossmann fold. NmrA shows an unexpected similarity to the short-chain dehydrogenase/reductase (SDR) family, with the closest relationship to UDP-galactose 4-epimerase. We show that NAD binds to NmrA, a previously unreported nucleotide binding property for this protein. NmrA is unlikely to be an active dehydrogenase, however, as the conserved catalytic tyrosine in SDRs is absent in NmrA, and thus the nucleotide binding to NmrA could have a regulatory function. Our results suggest that other transcription factors possess the SDR fold with functions including RNA binding. The SDR fold appears to have been adapted for other roles including non-enzymatic control functions such as transcriptional regulation and is likely to be more widespread than previously recognized.
Subject(s)
Repressor Proteins/chemistry , Repressor Proteins/metabolism , Transcription, Genetic , UDPglucose 4-Epimerase/chemistry , Amino Acid Sequence , Binding Sites , Catalytic Domain , Crystallography, X-Ray , Electrons , Fungal Proteins/chemistry , Models, Molecular , Molecular Sequence Data , NAD/metabolism , Neurospora crassa/enzymology , Protein Binding , Protein Folding , Protein Processing, Post-Translational , Protein Structure, Secondary , Protein Structure, Tertiary , Sequence Homology, Amino Acid , Transcription Factors/chemistry , Tyrosine/metabolismABSTRACT
The NmrA repressor protein of Aspergillus nidulans was overproduced in Escherichia coli and purified to homogeneity. Gel-exclusion chromatography showed that NmrA was monomeric in solution under the buffer conditions used. The protein was crystallized in three forms, belonging to trigonal, monoclinic and hexagonal space groups. Two of these crystal forms (A and B) diffract to high resolution and thus appear suitable for structure determination. Crystal form A belongs to space group P3((1))21, with unit-cell parameters a = b = 76.8, c = 104.9 A. Crystal form B belongs to space group C2, with unit-cell parameters a = 148.8, b = 64.3, c = 110.2 A, beta = 121.8 degrees.
