Search PubMed⌕ Search

Biomedical subjects

M I Simon

Publications and source records attributed to M I Simon.

At least 235 records · Page 13Linked to original sources
PubMed1983

Chromosomal mapping of the mink cell focus-inducing and xenotropic env gene family in the mouse.

Chromosomal locations of members of the xenotropic-related env gene family in the mouse genome have been determined. Endonuclease restriction site polymorphisms detected by molecular hybridization were used to study the inheritance of mink cell-focus inducing and xenotropic env gene-related sequences in recombinant inbred strains of mice. Some of the endogenous env sequences appear to be closely linked to genes determining leukemia virus induction and to genes involved in the immune response, such as the heavy and light chains of the immunoglobulin molecules or allotypic determinants on B and T lymphocytes. The use of probes that detect restriction fragment length polymorphisms in a small family of dispersed sequences promises to yield a large number of markers that can be used together with recombinant inbred strains for efficient mapping of the mouse genome.

Animals↗
PubMed1983

Primary structure of the M subunit of the reaction center from Rhodopseudomonas sphaeroides.

The reaction center is a membrane-bound bacteriochlorophyll-protein complex that mediates the primary photochemical events in the photosynthetic bacterium Rhodopseudomonas sphaeroides. The previously determined amino-terminal sequences of the three subunits of the reaction center protein were used to design synthetic mixed oligonucleotide probes for the structural genes encoding the subunits. One of these probes was used to isolate and clone a fragment of DNA from R. sphaeroides that contained the gene encoding the M subunit. The nucleotide sequence of this gene was determined by the dideoxy method. In addition, a number of tryptic and chymotryptic peptides from the M protein were isolated and subjected to sequence analysis, and the sequence of the carboxyl terminus was determined. Together with the amino-terminal sequence, the data establish the primary structure of the M protein. The distribution of hydrophobic residues in the amino acid sequence suggests the presence of five membrane-spanning segments. A significant homology was found between the amino acid sequence of the M subunit and a thylakoid membrane protein (M(r) 32,000) from spinach that has been implicated in herbicide and quinone binding.

Journal Article↗
PubMed1982

Genetic analysis of the mechanism of the Salmonella phase variation site specific recombination system.

Phase variation, the alternation of expression of flagellar antigens H1 and H2, in Salmonella typhimurium is mediated by site specific inversion of a 995 bp DNA segment of the chromosome. Hin, a protein encoded within the 995 bp segment, is thought to catalyze the recombination reaction between 14 bp inverted repeats flanking the 995 bp segment. By comparison of the relative rates of inversion of two different plasmids containing the H2 inversion segment flanked by different sequences, we conclude that the sequences adjacent to the inversion segment affect the rate of inversion. Homologous pairing of the repeats is important in H2 inversion since the orientation of the repeats on the host molecule(s) determines the result of the recombination reaction. The presence of the hin gene mediates the fusion of two plasmids when each contains one of the 14 bp repeat sequences. When the 14 bp sequences are direct repeats on a single molecule the sequence between them is deleted. These results support the hypothesis that the H2 inversion system functions by homologous, conservative, site specific recombination which is similar to the systems found associated with TnA transposons and temperate bacteriophage.

Bacterial Proteins↗
PubMed1982

Integral membrane proteins required for bacterial motility and chemotaxis.

Several of the gene products required for bacterial motility and chemotaxis are integral components of the cytoplasmic membrane. Amplification of the expression of one of these genes, motB, using in vitro recombinant DNA techniques, has allowed us to initiate a study of the assembly of this protein into the membrane. The identical sizes of the motB gene products synthesised in vivo and in vitro strongly suggest that assembly is achieved without the involvement of a proteolytically processed leader peptide. Further analysis of this system should yield valuable insights into the specific determinants of the cellular localisation of the motB gene product. Two other membrane proteins, the tsr and tar products have been studied in relation to their role as integrating channels for the transfer of sensory information into the bacterial cell. A model of the functional properties of these proteins is presented and discussed in terms of recent findings that they are subject to multiple covalent modifications during sensory adaptation.

Bacterial Proteins↗
PubMed1981

Diphtheria toxin forms transmembrane channels in planar lipid bilayers.

When exposed to a lipid bilayer, diphtheria toxin binds to it and forms transmembrane, voltage-dependent, anion-selective channels. The mean (+/- SD) conductance of these channels in asolectin membranes is 6.2 +/- 0.7 pmho (pS) in 0.2 M NaCl and 20 +/- 2 pmho in 1.0 M NaCl. The rate of channel formation depends on the pH in the toxin-containing compartment; it is very low at pH greater than 5.0 and increases abruptly as the pH decreases from 4.9 to 4.0. Binding of toxin to the membrane is also pH dependent, being unmeasurable at pH 7 and increasing monotonically with decreasing pH. The rate of channel formation depends upon membrane potential as well; channels form only at negative potentials. These channels are permanent in the time scale of the experiments (about 1 hr). The membrane conductance caused by the channels is also voltage dependent, being constant at positive potentials and decreasing at negative potentials. Hence, the current-voltage curve is linear at positive potentials and sublinear at negative potentials. The conditions necessary for insertion of toxin into the bilayer and formation of channels are similar to those that prevail inside the lysosome. Thus, these results lend credence to the idea that toxin enters the cytoplasm from the lysosomal compartment.

Anions↗
PubMed1980

The entry of diphtheria toxin into the mammalian cell cytoplasm: evidence for lysosomal involvement.

Lysosomotropic amines, such as ammonium chloride, are known to protect cells from the cytotoxic effects of diphtheria toxin. These drugs are believed to inhibit the transport of the toxin from a receptor at the cell exterior into the cytoplasm where a fragment of the toxin arrests protein synthesis. We studied the effects of lysosomotropic agents on the cytotoxic process to better understand how the toxin enters the cytoplasm. The cytotoxic effects of diphtheria toxin were not inhibited by antitoxin when cells were preincubated at 37 degrees C with toxin and ammonium chloride, exposed to antitoxin at 4 degrees C, washed to relieve the ammonium chloride inhibition, and finally warmed to 37 degrees C. The antigenic determinants of the toxin were, therefore, either altered or sheltered. It is likely that the combination of ammonium chloride and a low temperature trapped the toxin in an intracellular vesicle from which the toxin could proceed to the cytoplasm. Because lysosomotropic amines raise the pH within acidic intracellular vesicles, such as lysosomes, they could trap the toxin within such a vesicle if an acidic environment were necessary for the toxin to penetrate into the cytoplasm. We simulated acidic conditions which the toxin might encounter by exposing cells with toxin bound to their surface to acidic medium. We then measured the effects of lysosomotropic amines on the activity of the toxin to see if the acidic environment substituted for the function normally inhibited by the drugs. The drugs no longer protected the cells. This suggests that exposing the toxin to an acidic environment, such as that found within lysosomes, is an important step in the penetration of diphtheria toxin into the cytoplasm.

Ammonium Chloride↗
PubMed1980

Multiple electrophoretic forms of methyl-accepting chemotaxis proteins generated by stimulus-elicited methylation in Escherichia coli.

The tsr and tar genetic loci of Escherichia coli determine the presence in sodium dodecyl sulfate-polyacrylamide gel electrophoresis of methyl-accepting chemotaxis proteins (MCPs) I and II, respectively, each of which consists of a distinct group of multiple bands. Synthesis of the tsr and tar products was directed in ultraviolet-irradiated bacteria by lambda transducing phages. The addition of appropriate chemotactic stimuli to these cells resulted in the appearance of additional, faster migrating electrophoretic forms of the Tsr and Tar polypeptides which disappeared upon removal of the stimulus. The stimulus-elicited forms comigrated with component bands of the corresponding MCPs. These results indicate that methylation itself caused shifts in electrophoretic mobility and hence led to the observed MCP band patterns. The number of Tsr species suggested that there were at least three methylated sites on the Tsr polypeptide. The conclusion that methylation generates multiplicity was supported by the results of experiments in which the tsr product was synthesized in mutant bacteria defective in specific chemotaxis functions concerned with methylation or demethylation of MCPs. Thus, the presence of a cheX defect blocked the stimulus-elicited appearance of faster migrating forms of the tsr product; conversely, the presence of a cheB defect resulted in a pronounced shift toward these forms in the absence of a chemotactic stimulus.

Aspartic Acid↗
PubMed1979

Biochemical and genetic characterization of three hamster cell mutants resistant to diphtheria toxin.

We describe here three different hamster cell mutants which are resistant to diphtheria toxin and which provide models for investigating some of the functions required by the toxin inactivates elongation factor 2 (EF-2). Cell-free extracts from mutants Dtx(r)-3 was codominant. The evidence suggests that the codominant phenotype is the result of a mutation in a gene coding for EF-2. The recessive phenotype might arise by alteration of an enzyme which modifies the structure of EF-2 so that it becomes a substrate for reaction with the toxin. Another mutant, Dtx(r)-2, contained EF-2 that was sensitive to the toxin and this phenotype was recessive. Pseudomonas aeruginosa exotoxin is known to inactivate EF-2 as does diphtheria toxin and we tested the mutants for cross-resistance to pseudomonas exotoxin. Dtx(r)-1 and Dtx(r)-3 were cross-resistant while Dtx(r)-2 was not. It is known that diphtheria toxin does not penetrate to the cytoplasm of mouse cells and that these cell have a naturally occurring phenotype of diphtheria toxin resistance. We fused each of the mutants with mouse 3T3 cells and measured the resistance. We fused each of the mutants with mouse 3T3 cells and measured the resistance of the hybrid cells to diphtheria toxin. Intraspecies hybrids containing the genome of mutants Dtx(r)-1 and Dtx(r)-3 had some resistance while those formed with Dtx(r)-2 were as sensitive as hybrids derived from fusions between wild-type hamster cells and mouse 3T3 cells.

Animals↗
PubMed1978

Studies of the diphtheria toxin receptor on Chinese hamster cells.

Concanavalin A, wheat germ agglutinin and the ovalbumin glycopeptide are all inhibitors of the cytotoxic effect of diphtheria toxin on Chinese hamster cells. Ovalbumin glycopeptide loses its inhibitory property after treatment with beta-N-acetylglucosaminidase. This demonstrates the importance of the glycopeptide structure for the mechanism of inhibition. The glycopeptide may be a toxin cell-surface receptor analogue. Diphtheria toxin-resistant mutants were isolated in order to search for cells that might have an altered toxin receptor. One mutant was 10- to 15-fold more resistant to diphtheria toxin than wild-type cells when protein synthesis was measured as a function of toxin concentration. However, when protein synthesis was measured as a function of time at a high toxin concentration, the time before onset of inhibition was identical in the mutant and wild-type cells. We present evidence indicating that the resistance of this mutant can be accounted for by a decreased affinity of toxin for a cell-surface receptor.

Cell Line↗
PubMed1978

Diphtheria toxin has the properties of a lectin.

The inhibition of protein synthesis in Chinese hamster V79 cells by diphtheria toxin is antagonized by the lectins concanavalin A, succinylated concanavalin A, and wheat germ agglutinin but not by Proteus vulgaris phytohemagglutinin or abrus agglutinin. The effects of concanavalin A and wheat germ agglutinin are reversed by methyl alpha-mannoside and N-acetylglucosamine, respectively. The inhibition of diphtheria toxin as a function of concanavalin A concentration fits a model of competitive inhibition with an apparent dissociation constant for concanavalin A of 3 X 10(-8) M. These results suggest that the diphtheria toxin receptor may be an oligosaccharide. To test this hypothesis, we screened several oligosaccharides for the ability to inhibit diphtheria toxin. The cell wall polysaccharide of Salmonella cholera suis and the ovalbumin glycopeptide were effective inhibitors. These studies suggest that diphtheria toxin may have the oligosaccharide binding properties of a lectin with specificity for N-acetylglucosamine and mannose.

Cell Line↗
PubMed1977

Classification of Bacillus subtilis flagellins.

Purified flagellins derived from 16 strains of Bacillus subtilis were classified into at least five distinct groups on the basis of their reaction with antiflagellar filament antibody and antiflagellin antibody. This classification was in good accord with that derived independently on the basis of amino acid analyses of the flagellins. Flagellar antigenicity appears to provide a useful typological character in classifying B. subtilis strains.

Amino Acids↗
PubMed1977

Localization of proteins controlling motility and chemotaxis in Escherichia coli.

Flagellar proteins controlling motility and chemotaxis in Escherichia coli were selectively labeled in vivo with [35S]methionine. This distribution of these proteins in subcellular fractions was examined by sodium dodecyl sulfatepolyacrylamide gel electrophoresis and autoradiography. The motA, motB, cheM, and cheD gene products were found to be confined exclusively to the inner cytoplasmic membrane fraction, whereas the cheY, cheW, and cheA (66,000 daltons) polypeptides appeared only in the soluble cytoplasmic fraction. The cheB, cheX, cheZ, and cheA (76,000 daltons) proteins, however, were distributed in both the cytoplasm and the inner membrane fractions. The hag gene product (flagellin) was the only flagellar protein examined that copurified with the outer lipopolysaccharide membrane. Differences in the intracellular locations of the che and mot gene prodcuts presumably reflect the functional attributes of these components.

Bacterial Proteins↗
PubMed1977

Bacterial flagella.

Explore the source record for details and available documents.

Antigens, Bacterial↗
PubMed1972

Flagellar assembly mutants in Escherichia coli.

Genetic and biochemical analysis of mutants defective in the synthesis of flagella in Escherichia coli revealed an unusual class of mutants. These mutants were found to produce short, curly, flagella-like filaments with low amplitude ( approximately 0.06 mum). The filaments were connected to characteristic flagellar basal caps and extended for 1 to 2 mum from the bacterial surface. The mutations in these strains were all members of one complementation group, group E, which is located between his and uvrC. The structural, serological, and chemical properties of the filament derived from the mutants closely resemble those of the flagellar hook structure. On the basis of these properties, it is suggested that these filaments are "polyhooks", i.e., repeated end-to-end polymers of the hook portion of the flagellum. Polyhooks are presumed to be the result of a defective cistron which normally functions to control the length of the hook region of the flagellum.

Alkanesulfonates↗
PubMed1971

Variation in the primary structure of Bacillus subtilis flagellins.

The flagella derived from 18 strains Bacillus subtilis were tested for their reaction with antiflagellar filament antibody and antiflagellin antibody. On the basis of their reactivity, at least five serologically distinct classes could be identified. Peptide map analysis of tryptic digests of the subunit proteins were consistent with the immunochemical analysis. Large differences in sequence existed among proteins of the different classes; proteins within an antigenic group differed by only a few peptides. Furthermore, 9 of the 27 tryptic peptides resolved were common to flagellin proteins from all the classes examined. The relationship between antigenic specificity, variability in peptide pattern, and the conformation of the flagellin protein are discussed.

Animals↗