293,340 research outputs found
Multiple binding sites for transcriptional repressors can produce regular bursting and enhance noise suppression
Cells may control fluctuations in protein levels by means of negative
autoregulation, where transcription factors bind DNA sites to repress their own
production. Theoretical studies have assumed a single binding site for the
repressor, while in most species it is found that multiple binding sites are
arranged in clusters. We study a stochastic description of negative
autoregulation with multiple binding sites for the repressor. We find that
increasing the number of binding sites induces regular bursting of gene
products. By tuning the threshold for repression, we show that multiple binding
sites can also suppress fluctuations. Our results highlight possible roles for
the presence of multiple binding sites of negative autoregulators
Nickel binding sites in histone proteins
Nickel compounds are well known as human carcinogens, though the molecular events that are responsible for this are not well understood.
It has been proposed that a crucial element in the mechanism of carcinogenesis is the binding of Ni(II) ions within the cell nucleus. It is known that DNA polymer binds Ni(II) only weakly, leaving the proteins of the cell nucleus as the likely Ni(II) targets. Being histone proteins the most abundant among them, they can be considered the primary sites for nickel binding. Here we describe the interactions of nickel with histone H4, core tetramer (H3-H4)2 and several peptide fragments which have been selected as the candidates for specific binding sites in the histone octamer. The results allowed us to propose several mechanisms of nickel induced damage resulting from metal coordination, including structural changes of histone proteins, as well as nucleobase oxidation and sequence-specific histone hydrolysis.
The aim of the present work is to provide a comprehensive overview of literature dealing with nickel coordination to histone proteins and its link with nickel involvement in toxicity and carcinogenicity
The evolution of complex gene regulation by low specificity binding sites
Transcription factor binding sites vary in their specificity, both within and
between species. Binding specificity has a strong impact on the evolution of
gene expression, because it determines how easily regulatory interactions are
gained and lost. Nevertheless, we have a relatively poor understanding of what
evolutionary forces determine the specificity of binding sites. Here we address
this question by studying regulatory modules composed of multiple binding
sites. Using a population-genetic model, we show that more complex regulatory
modules, composed of a greater number of binding sites, must employ binding
sites that are individually less specific, compared to less complex regulatory
modules. This effect is extremely general, and it hold regardless of the
regulatory logic of a module. We attribute this phenomenon to the inability of
stabilising selection to maintain highly specific sites in large regulatory
modules. Our analysis helps to explain broad empirical trends in the yeast
regulatory network: those genes with a greater number of transcriptional
regulators feature by less specific binding sites, and there is less variance
in their specificity, compared to genes with fewer regulators. Likewise, our
results also help to explain the well-known trend towards lower specificity in
the transcription factor binding sites of higher eukaryotes, which perform
complex regulatory tasks, compared to prokaryotes
Concerted Formation of Macromolecular \u3cem\u3eSuppressor-mutator\u3c/em\u3e Transposition Complexes
Transposition of the maize Suppressor-mutator (Spm) transposon requires two element-encoded proteins, TnpA and TnpD. Although there are multiple TnpA binding sites near each element end, binding of TnpA to DNA is not cooperative, and the binding affinity is not markedly affected by the number of binding sites per DNA fragment. However, intermolecular complexes form cooperatively between DNA fragments with three or more TnpA binding sites. TnpD, itself not a sequence-specific DNA-binding protein, binds to TnpA and stabilizes the TnpA-DNA complex. The high redundancy of TnpA binding sites at both element ends and the protein-protein interactions between DNA-bound TnpA complexes and between these and TnpD imply a concerted transition of the element from a linear to a protein crosslinked transposition complex within a very narrow protein concentration range
Receptor sites involved in posttranslational transport of apocytochrome c into mitochondria
Assembly of cytochrome c involves a series of steps: synthesis of apocytochrome c on free ribosomes, specific binding of apocytochrome c to the mitochondrial surface, transfer across the outer membrane, covalent addition of protoheme, refolding of the polypeptide chain, and association of holocytochrome c with its functional sites at the inner membrane. The binding step of apocytochrome c to Neurospora crassa mitochondria was studied by inhibiting the subsequent transfer steps with the heme analogue deuterohemin. The binding sites are highly specific for mitochondrial apocytochromes c. Bound labeled Neurospora apocytochrome c was competitively displaced by unlabeled apocytochrome c from various species. These exhibited different abilities for displacement. Apocytochrome c from Paracoccus denitrificans, the amino-terminal (heme-binding) fragment of Neurospora apocytochrome c, and Neurospora holocytochrome c did not recognize the binding sites. Polylysine did not interfere with apocytochrome c binding. Apocytochrome c is reversibly bound. The binding sites are present in limited number. High-affinity binding sites were present at about 90 pmol/mg of mitochondrial protein. They displayed an association constant of 2.2 X 10(7) M-1. Apocytochrome c was imported into mitochondria and converted to holocytochrome c directly from the binding sites when inhibition by deuterohemin was relieved. We conclude that the apocytochrome c binding sites on mitochondria represent receptors that function in the recognition and import of this precursor by mitochondria
Adaptive evolution of transcription factor binding sites
The regulation of a gene depends on the binding of transcription factors to
specific sites located in the regulatory region of the gene. The generation of
these binding sites and of cooperativity between them are essential building
blocks in the evolution of complex regulatory networks. We study a theoretical
model for the sequence evolution of binding sites by point mutations. The
approach is based on biophysical models for the binding of transcription
factors to DNA. Hence we derive empirically grounded fitness landscapes, which
enter a population genetics model including mutations, genetic drift, and
selection. We show that the selection for factor binding generically leads to
specific correlations between nucleotide frequencies at different positions of
a binding site. We demonstrate the possibility of rapid adaptive evolution
generating a new binding site for a given transcription factor by point
mutations. The evolutionary time required is estimated in terms of the neutral
(background) mutation rate, the selection coefficient, and the effective
population size. The efficiency of binding site formation is seen to depend on
two joint conditions: the binding site motif must be short enough and the
promoter region must be long enough. These constraints on promoter architecture
are indeed seen in eukaryotic systems. Furthermore, we analyse the adaptive
evolution of genetic switches and of signal integration through binding
cooperativity between different sites. Experimental tests of this picture
involving the statistics of polymorphisms and phylogenies of sites are
discussed.Comment: published versio
The iron binding-sites of chicken ovotransferrin
We have shown previously that the EXAFS spectrum of diferric chicken ovotransferrin (Fe2COT) can only be adequately simulated assuming a split first shell co-ordination [1]. EXAFS and XANES spectra of Fe2COT measured in solution and as a freeze-dried powder provide evidence for perturbation of the iron-binding sites on freeze-drying which involves the loss of one of the long (~2.04 Ă
) first shell ligands (presumably water). Measurement of the XANES of the C-terminal monoferric COT and a C-terminal domain fragment suggests that the metal binding site remains largely unperturbed by the fragmentation process. The possibility of site interaction is briefly discussed.We gratefully acknowledge the SERC for financial support and provision of facilities
Effect of thiols on beta 2-adrenoceptors in human mononuclear leucocytes
The effect of the disulfide reducing agent dithiothreitol (DTT) and other thiols on binding of the beta-adrenoceptor antagonist (-)-125iodocyanopindolol (125ICYP) to human mononuclear leucocytes (MNL) was investigated. Saturation experiments and dissociation kinetics revealed two classes of specific 125ICYP binding sites, one of high and the other of low affinity, respectively. In intact MNL DTT caused a decrease in specific binding. This was due almost selectively to a decrease in the affinity of high affinity binding sites, which decreased gradually in a concentration-dependent manner to the affinity of low affinity binding sites. In MNL membranes DTT decreased not only the affinity but also the number of high affinity binding sites. The DTT effect was completely reversible by simple reoxidation on air. The structural isomers (+/-)-DTT. (-)-DTT and dithioerythritol revealed identical effects on specific binding, whereas the monothiols mercaptoethanol and alpha-monothioglycerol, having a lower redox potential, were considerably less effective. In the same concentration range that influenced specific binding. DTT stimulated intracellular cAMP production. These results suggest functionally important disulfide bridges which regulate the affinity of beta-adrenoceptor binding sites in human MNL. They stabilize the receptor in a high affinity state; their reduction causes the conversion of the high affinity state into a low affinity state in a process associated with stimulation of adenylate cyclase. Available evidence indicates that a similar transformation is made by beta-adrenoceptor agonists. Consequently low affinity 125ICYP binding sites preexistent in untreated cells could represent a reduced receptor state resulting from agonist-receptor interaction in vivo
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