In Escherichia coli the levels of numerous enzymes associated with aerobic metabolism are decreased during anaerobic growth. In an arcA mutant the anaerobic levels of these enzymes are increased. The enzymes, which are encoded by different regulons, include members that belong to the tricarboxylic acid cycle, the glyoxylate shunt, the pathway for fatty acid degradation, several dehydrogenases of the flavoprotein class, and the cytochrome o oxidase complex. Transductional crosses placed the arcA gene near min o on the chromosomal map. Complementation tests showed that the arcA gene corresponded to the dye gene, which is also known as fexA, msp, seg, or sfrA because of various phenotypic properties [Bachmann, B. (1983) Microbiol. Rev. 47, 180-230].A dye-deletion mutant was derepressed in the aerobic enzyme system. The term modulon is proposed to describe a set of regulons that are subject to a common transcriptional control.In facultative anaerobes such as Escherichia coli, the network for electron transport is organized in such a way that the energetically most favorable pathway is used. On the basis of the regulatory patterns of gene expression, these networks appear to be three main hierarchical systems. The aerobic respiratory pathways conduct the flow of electrons (or reducing equivalents) to 02 (E"' = + 0.82 V). In the absence of 02, nitrate (E"' = + 0.42 V) can be used. In this first system ubiquinone (E"' = +0.1 V) acts as an adapter and passes the electrons to the 02 or nitrate acceptor chain.The next preferred system conducts electrons to terminal acceptors such as trimethylamine N-oxide (E ' = + 0.13 V) or fumarate (E"' = +0.03 V), with menaquinone (E"' = -0.074 V) as the adapter. Both respiratory pathways supply the cell with metabolic energy by generating proton-motive force across the cytoplasmic membrane. As the operation of these pathways is curtailed by the shortage of exogenous electron acceptors, the cell relies increasingly on the energetically least rewarding system, in which redox reactions are balanced by fermentative dismutation of carbon and energy sources. The pyridine nucleotides serve as the adapter for the transfer of electrons, and substrate-level phosphorylation becomes the principal means of energy generation. The choice of the redox pathways is biased in favor of those that provide the greatest difference in midpoint potential between the initial electron donor and the terminal acceptor, for this difference limits the amount of energy that can be harnessed (for reviews see refs. 1 and 2).The flow of electrons through the three main redox systems and the channeling within each is guided by a complex set of controls that include the regulation of gene expression. Thus, 02 respiration interferes with induction of the anaerobic respiratory system. The mechanism of this control is beginning to be understood. (4). The apo-repressor may be encoded by the narL gene (5), which also acts as an activator for the synthesis of nitrate reductase (6). The mechanisms by which enzymes that...
C2H2 zinc finger proteins probably comprise the largest family of regulatory proteins in mammals. Most zinc fingers bind to a cognate DNA. In addition to DNA, many of the proteins also bind to RNA or protein, and some bind to RNA only. The binding properties depend on the amino acid sequence of the finger domains and of the linker between fingers, as well as on the higher-order structures and the number of fingers. C2H2 zinc finger proteins contain from 1 to more than 30 figures. Based on the number and the pattern of the fingers, most of the proteins can be classified into one of three groups: triple-C2H2, multiple-adjacent-C2H2, and separated-paired-C2H2 finger proteins. In contrast to proteins with triple-C2H2 fingers, proteins with multiple-adjacent-C2H2 fingers can bind multiple, different ligands. Proteins with a number of separated-paired fingers bind to the target by means of only a single pair.
The arcA (dye) and arcB genes of Escherichia coli are responsible for anaerobic repression of target operons and regulons of aerobic function (the arc modulon). The amino acid sequence of ArcA (Dye) indicated that it is the regulator protein of a two-component control system. Here we show that ArcB is a membrane sensor protein on the basis of its deduced amino acid sequence (778 residues), hydropathicity profile, and cellular distribution. On the carboxyl end of the ArcB sequence there is an additional domain showing homology with conserved regions of regulator proteins. Deletion into this domain destroyed ArcB function. ArcB conserved a histidine residue for autophosphorylation of the sensor proteins, and aspartic residues important for the regulator proteins.
Human embryonic stem cells injected into scid mice produce nodules containing differentiated somatic tissues. From the trypsinized cells of such a nodule, we have recovered keratinocytes that can be grown in cell culture. The method of recovery is sensitive enough to detect small numbers of keratinocytes formed in the nodule, but for purposes of analysis, it is preferable to study the development of the entire keratinocyte lineage in culture. The principle of our analysis is the successive appearance of markers, including transcription factors with considerable specificity for the keratinocyte (p63 and basonuclin) and differentiation markers characteristic of its final state (keratin 14 and involucrin). We have determined the order of marker succession during the time-and migration-dependent development of keratinocytes from single embryoid bodies in cell culture. Of the markers we have examined, p63 was the earliest to appear in the keratinocyte lineage. The successive accumulation of later markers provides increasing certainty of emergence of the definitive keratinocyte.
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