Epigenetics refers to heritable patterns of gene expression that do not depend on alterations of the genomic DNA sequence. Nickel compounds have demonstrated carcinogenicity without any associated mutagenesis, suggesting that its mechanism of carcinogenesis is epigenetic in nature. One such potential mechanism is the heterochromatinization of chromatin within a region of the genome containing a gene sequence, inhibiting any further molecular interactions with that underlying gene sequence and effectively inactivating that gene. We report here the observation, by atomic force microscopy and circular dichroism spectropolarimetry, that nickel ion (Ni 2+ ) condenses chromatin to a greater extent than the natural divalent cation of the cell, magnesium ion (Mg 2+ ). In addition, we use a model experimental system that incorporates a transgene, the bacterial xanthine guanine phosphoribosyl transferase gene (gpt) differentially near to, and far away from, a heterochromatic region of the genome, in two cell lines, the Chinese hamster V79-derived G12 and G10 cells, respectively, to demonstrate by DNase I protection assay that nickel treatement protects the gpt gene sequence from DNase I exonuclease digestion in the G12 cells, but not in the G10 cells. We conclude that condensation of chromatin by nickel is a potential mechanism of nickel-mediated gene regulation. Keywords chromatin; heterochromatin; nickel; divalent cation; carcinogenesisChromatin is the natural state of protein-associated DNA in all eukaryotes. It is a dynamic polymer of subunits called nucleosomes. The nucleosome subunit consists of approximately 146 bps of DNA wrapped around an octamer of histone proteins. The histone octamer is made up of two each of histone H2A, H2B, H3, and H4. The DNA makes one and a half turns around the octamer, with the entry and exit points of the DNA at opposite sides of the disk-shaped histone octameric complex. This complex of 146 bps and the octamer of histones is known as the nucleosome core particle. In chromatin, the exiting and entering DNA duplex strands of this nucleosome core particle extend to the next nucleosome core particles on either side of it. The intervening DNA connecting each pair of nucleosomes is termed the linker DNA. The lengths of these intervening DNA duplex strands vary from species to species and from tissue types to tissues types, as well as from one stretch between two nucleosomes to another. Linker DNA length ranges from a minimum of 25 bps to upwards of 100 bps, averaging approximately 50 to 60 bps (1). A fifth histone type, the linker histone (H1), binds to the linker DNA in a stoichiometry of 1 H1 to 1 nucleosome. The complex of the core particle (the histone octamer

A procedure is described for selection of temperature-sensitive mutants affecting fatty-acid synthesis based upon radiation suicide of wild-type organisms by tritiated acetate selectively incorporated into fatty acids. At 370, two of the mutants extensively incorporate fatty-acid supplements provided in the medium, and grow for extended periods only when a trans-unsaturated or a combination of saturated and cis-unsaturated fatty acids is available. In vivo fatty-acid synthesis, measured by ['4Cjacetate incorporation, is temperature-sensitive in these strains relative to protein synthesis and other nonlipid macromolecular syntheses using acetate. The biochemical nature of these mutations has not been identified.Saturated, unsaturated, and ,3-hydroxy fatty acids of Escherichia coli are synthesized in part by a common pathway. From the known sequence of reactions, one might expect to isolate mutants blocked in total fatty-acid synthesis or selectively defective in the synthesis of one or more classes of fatty acid. Unsaturated fatty-acid auxotrophs have been isolated by exogenous replacement of the essential fatty acid and penicillin enrichment (1-3). Recovery of such mutants is low, because under conditions of fatty-acid starvation (used in penicillin enrichment) the unsaturated fatty-acid auxotrophs continue to grow for one generation, and then begin to die (2). Furthermore, this approach, which relies on replacement, has not yielded any other class of fatty-acid biosynthetic (fab) mutants. Mutations affecting the synthesis of these other types of fatty acids may not be critical to cells or, on the other hand, might be lethal. For either reason they would not ordinarily be isolated. Recently, conditional mutants for phospholipid biosynthesis were isolated because they failed to assimilate a radioactive, and specific, phospholipid precursor and, thus, were protected from radiation death undergone by cells with an intact biosynthetic pathway (4). In this paper, we report conditions for selectively labeling cellular fatty acids with tritiated acetate of high specific activity and isolating,'conditional mutants affecting fatty-acid synthesis. Such mutants survive better than wild-type cells because they do not concentrate the radioisotope. Properties of two of these mutants are described. MATERIALS AND METHODSStrains and Media. All bacterial strains were K-12 strains of E. coli. The mutants obtained in this study were derived from AB1623, a glutamate-requiring strain defective in citrate synthetase, which was provided by Dr. H. L. Kornberg and has been described (5). LA1-6 and LA2-22 are two independently isolated mutants thermosensitive for fatty-acid biosynthesis; LA1-6-2 and LA2-22-6 are wild-type transductants of these mutants. In the symbols adopted for classifying mutants, L indicates lipid, A identifies the genetic background (AB1623), the numeral before the hyphen refers to the particular mutant selection, and the number after the hyphen is the particular isolate obtained. C600 is F-and requires leuci...

A new method for analyzing three-state protein unfolding equilibria is described that overcomes the difficulties created by direct effects of denaturants on circular dichroism (CD) and fluorescence spectra of the intermediate state. The procedure begins with a singular value analysis of the data matrix to determine the number of contributing species and perturbations. This result is used to choose a fitting model and remove all spectra from the fitting equation. Because the fitting model is a product of a matrix function which is nonlinear in the thermodynamic parameters and a matrix that is linear in the parameters that specify component spectra, the problem is solved with a variable projection algorithm. Advantages of this procedure are perturbation spectra do not have to be estimated before fitting, arbitrary assumptions about magnitudes of parameters that describe the intermediate state are not required, and multiple experiments involving different spectroscopic techniques can be simultaneously analyzed. Two tests of this method were performed: First, simulated three-state data were analyzed, and the original and recovered thermodynamic parameters agreed within one standard error, whereas recovered and original component spectra agreed within 0.5%. Second, guanidine-induced unfolding titrations of the human retinoid-X-receptor ligandbinding domain were analyzed according to a three-state model. The standard unfolding free energy changes in the absence of guanidine and the guanidine concentrations at zero free-energy change for both transitions were determined from a joint analysis of fluorescence and CD spectra. Realistic spectra of the three protein states were also obtained.Keywords: three-state protein unfolding equilibria; global analysis method; denaturant perturbation; separable least squares; macrophage colony-stimulating factor; retinoid receptor Equilibrium titrations of protein state as a function of denaturant concentration are a valuable means of identifying the minimal set of intermediates in mechanistic models of folding/unfolding pathways, and of measuring parameters that describe the thermodynamic stabilities of the native protein and any intermediates that are present. The two techniques most commonly used to characterize and quantitate native and unfolded protein states in solution are far-UV circular dichroism (CD) and fluorescence spectroscopies, which are complementary because CD spectra at wavelengths below 250 nm are a measure of protein secondary structure (Greenfield 1996;Johnson Jr. 1999), whereas fluorescence spectra are responsive to the environment of tryptophan and tyrosine residues (Lakowicz 1999). If intermediate states are also present in the equilibrium mixture, it is likely that they will differ from the native and unfolded states by their respective secondary structure contents and/or fluorophore environments. Therefore, an unfolding titration should be monitored by both forms of spectroscopy as a means of detecting and measuring these intermediates. In addition, u...

Both multivalent ions and 85% ethanol are required to produce the original Z' form of poly(dGdC):poly(dGdC) in solution. When multivalent impurities are removed by dialysis against 0.5 M NaCl and 1 mM EDTA, the circular dichroism retains features of the standard Z form. Addition of Ca+2 nearly reverses this effect. Analysis by singular value decomposition of near-ultraviolet circular dichroism spectra collected during titrations of polynucleotide in 60% ethanol with multivalent ions reveal that, at concentrations below .5 per nucleotide, they stabilize Z'-like forms in a two-state equilibrium with the Z form. Differences among the Z' spectra produced by the different ions suggest that at least three families of Z' structures exist. Furthermore, comparison with crystal data indicates that the Z' form in solution is related to the ZII form in crystals.

Both researchers and practitioners are only in the early stages of examining and understanding the application of artificial intelligence (AI) in terms of marketing themselves as employers or the open jobs they have. AI has the potential to significantly affect how firms reach, identify, attract, and select human capital. We examine factors that can influence a job candidate's intent to complete AI-enabled recruiting processes, especially the influence of a firm's use of biometrics in that process. The results show that (1) social media can increase technology use motivation and AI-enabled recruiting with (2) trendiness as a first stage boundary condition and (3) biometrics as a second stage boundary condition. We contribute to marketing knowledge by identifying that for managers wanting to influence job seekers’ technology use motivation in order to increase their participation in AI-enabled recruiting; they must focus on the indirect effects of trendiness, biometrics, and their social media usage.