Showing papers in "Proceedings of the National Academy of Sciences of the United States of America in 1972"
TL;DR: A convenient technique for the partial purification of large quantities of functional, poly(adenylic acid)-rich mRNA is described and should prove generally useful as an initial step in the isolation of specific mRNAs.
Abstract: A convenient technique for the partial purification of large quantities of functional, poly(adenylic acid)-rich mRNA is described. The method depends upon annealing poly(adenylic acid)-rich mRNA to oligothymidylic acid-cellulose columns and its elution with buffers of low ionic strength. Biologically active rabbit globin mRNA has been purified by this procedure and assayed for its ability to direct the synthesis of rabbit globin in a cell-free extract of ascites tumor. Inasmuch as various mammalian mRNAs appear to be rich in poly(adenylic acid) and can likely be translated in the ascites cell-free extract, this approach should prove generally useful as an initial step in the isolation of specific mRNAs.
5,833 citations
TL;DR: Covalently-closed, catenated, and open (nicked) circular forms of R-factor DNA are all effective in transformation, but denaturation and sonication abolish the transforming ability of R.factor DNA in this system.
Abstract: Transformation of E. coli cells treated with CaCl2 to multiple antibiotic resistance by purified R-factor DNA is reported. Drug resistance is expressed in a small fraction of the recipient bacterial population almost immediately after uptake of DNA, but full genetic expression of resistance requires subsequent incubation in drugfree medium before antibiotic challenge. Transformed bacteria acquire a closed circular, transferable DNA species having the resistance, fertility, and sedimentation characteristics of the parent R factor. Covalently-closed, catenated, and open (nicked) circular forms of R-factor DNA are all effective in transformation, but denaturation and sonication abolish the transforming ability of R-factor DNA in this system.
2,907 citations
TL;DR: Bilateral electrolytic lesions in the suprachiasmatic nuclei permanently eliminated nocturnal and circadian rhythms in drinking behavior and locomotor activity of albino rats.
Abstract: Bilateral electrolytic lesions in the suprachiasmatic nuclei permanently eliminated nocturnal and circadian rhythms in drinking behavior and locomotor activity of albino rats. The generation of 24-hr behavioral rhythms and the entrainment of these rhythms to the light-dark cycle of environmental illumination may be coordinated by neurons in the suprachiasmatic region of the rat brain. Destruction of the medial preoptic area had no effect on 24-hr drinking rhythms.
1,990 citations
TL;DR: There is a significant difference between the capacity of bilayers made from mono-layers and that of hydrocarbon-containing bilayer made by phase transition; the average values are 0.9 and 0.45 muF cm(-2), respectively, which approximates that of biological membranes.
Abstract: Bimolecular membranes are formed from two lipid monolayers at an air-water interface by the apposition of their hydrocarbon chains when an aperture in a Teflon partition separating two aqueous phases is lowered through the interface. Formation of the membrane is monitored by an increase of the electrical capacity, as measured with a voltage clamp. Electrical resistance of the unmodified membrane is analogous to that of conventional planar bilayers (black lipid membranes) prepared in the presence of a hydrocarbon solvent, i.e., 106-108 ohm cm2; the resistance can be lowered to values of 103 ohm cm2 by gramicidin, an antibiotic that modifies the conductance only when the membranes are of biomolecular thickness. In contrast to the resistance, there is a significant difference between the capacity of bilayers made from mono-layers and that of hydrocarbon-containing bilayers made by phase transition; the average values are 0.9 and 0.45 μF cm-2, respectively. The value of 0.9 μF cm-2 approximates that of biological membranes. Assuming a dielectric constant of 2.1 for the hydrocarbon region, the dielectric thickness, as calculated from a capacity of 0.9 μF cm-2, is 22 A. This value is 6-10 A smaller than the actual thickness of the hydrocarbon region of bilayers and cell membranes, as determined by x-ray diffraction. The difference may be due to a limited penetration of water into the hydrocarbon region near the ester groups that would lower the electrical resistance of this region and reduce the dielectric thickness. Asymmetric membranes have been formed by adjoining two lipid monolayers of different chemical composition.
1,668 citations
TL;DR: The results suggest that dopamine-sensitive adenylate cyclase may be the receptor for dopamine in mammalian brain and should facilitate the search for new therapeutic agents useful in the treatment of extrapyramidal diseases.
Abstract: An adenylate cyclase that is activated specifically by low concentrations of dopamine has been demonstrated in homogenates of caudate nucleus of rat brain. A half-maximal increase in the activity of the enzyme occurred in the presence of 4 μM dopamine. Concentrations of dopamine as low as 0.3 μM stimulated the activity of the enzyme. The adenylate cyclase activity of the homogenates was also stimulated by low concentrations of apomorphine, a substance known to mimic the physiological and pharmacological effects of dopamine. The stimulatory effect of dopamine was blocked by low concentrations of either haloperidol or chlorpromazine, agents known to block the actions of dopamine in mammalian brain. The results suggest that dopamine-sensitive adenylate cyclase may be the receptor for dopamine in mammalian brain. The isolation of this enzyme from caudate nucleus should facilitate the search for new therapeutic agents useful in the treatment of extrapyramidal diseases.
968 citations
TL;DR: It was found, however, that a sudden increase also elicits a response, namely supercoordinated swimming, which demonstrates that chemotaxis is achieved by modulation of the incidence of tumbling both above and below its steady-state value.
Abstract: A “temporal gradient apparatus” has been developed that allows the motility of bacteria to be studied after they have been subjected to a sudden change from one uniform concentration of attractant to another. A sudden decrease elicits the tumbling response observed with spatial gradients; it was found, however, that a sudden increase also elicits a response, namely supercoordinated swimming. This demonstrates that chemotaxis is achieved by modulation of the incidence of tumbling both above and below its steady-state value. The initial responses gradually revert to the steady-state motility pattern characteristic of a uniform distribution of attractant. The apparent detection of a spatial gradient by the bacteria therefore involves an actual detection of a temporal gradient experienced as a result of movement through space. Potential models for the chemotactic response based on some “memory” mechanism are discussed.
869 citations
TL;DR: Coarse powders of acid-insoluble matrix of diaphysis and calvarial parietal bone rapidly and consistently transformed fibroblasts into masses of cartilage and bone containing hemopoietic marrow.
Abstract: Coarse powders of acid-insoluble matrix of diaphysis and calvarial parietal bone rapidly and consistently transformed fibroblasts into masses of cartilage and bone containing hemopoietic marrow. The transformant was encapsulated by fibroblasts within 24 hr to form a plaque. Transformation was restricted to the central thicknesses of the plaque. Under the stated conditions the alteration of the phenotype, fibroblast to chondroblast, was an unstable transformation, whereas the phenotype change, fibroblast to osteoblast, was stable. The transformation occurred on a rigid timetable of sequences. Measurements of alkaline phosphatase activity and incorporation of radioactive sulfate, phosphate, and calcium were sensitive and quantitative assays for the appearance of the transformed products, cartilage and bone.
816 citations
TL;DR: Methods for covalently joining duplex DNA molecules to one another are developed and used to construct circular dimers of SV40 DNA and to insert a DNA segment containing lambda phage genes and the galactose operon of E. coli into SV40DNA.
Abstract: We have developed methods for covalently joining duplex DNA molecules to one another and have used these techniques to construct circular dimers of SV40 DNA and to insert a DNA segment containing lambda phage genes and the galactose operon of E coli into SV40 DNA The method involves: (a) converting circular SV40 DNA to a linear form, (b) adding single-stranded homodeoxypolymeric extensions of defined composition and length to the 3′ ends of one of the DNA strands with the enzyme terminal deoxynucleotidyl transferase (c) adding complementary homodeoxypolymeric extensions to the other DNA strand, (d) annealing the two DNA molecules to form a circular duplex structure, and (e) filling the gaps and sealing nicks in this structure with E coli DNA polymerase and DNA ligase to form a covalently closed-circular DNA molecule
673 citations
TL;DR: The results show that genes determining neurotransmitter species can be expressed in dividing cells, that the parental programs of gene expression are inherited, and that dividing cells can be programmed with respect to their ability to communicate with other cells.
Abstract: Neuroblastoma clones were examined for choline acetyltransferase (EC 2.3.1.6), tyrosine hydroxylase (EC 1.14.3.a), acetylcholinesterase (EC 3.1.1.7), and also for neurite formation. One clone does not form axons or dendrites. Three types of clones were found with respect to neurotransmitter synthesis: cholinergic, adrenergic, and clones that do not synthesize acetylcholine or catechols. All clones contain acetylcholinesterase. These results show that genes determining neurotransmitter species can be expressed in dividing cells, that the parental programs of gene expression are inherited, and that dividing cells can be programmed with respect to their ability to communicate with other cells.
643 citations
TL;DR: The staphylococcal protease hydrolyzes all of the seventeen different glutamoyl bonds studied, although those involving hydrophobic aminoacid residues with bulky side chains are cleaved at a lower rate.
Abstract: An extracellular protease of Staphylococcus aureus, strain V8, previously shown to cleave specifically the peptide bonds on the carboxyl-terminal side of either aspartate or glutamate residues in phosphate buffer (pH 7.8) hydrolyzes only glutamoyl bonds in either ammonium bicarbonate (pH 7.8) or ammonium acetate (pH 4.0). Of all aspartoyl bonds tested, only the Asp-Gly linkage is cleaved at a detectable rate. The staphylococcal protease hydrolyzes all of the seventeen different glutamoyl bonds studied, although those involving hydrophobic aminoacid residues with bulky side chains are cleaved at a lower rate.
640 citations
TL;DR: There is an effective limit to niche overlap in the real world, and this limit is insensitive to the degree of environmental fluctuation, unless it be very severe.
Abstract: The relationship between environmental variability and niche overlap is studied for a class of model biological communities in which several species compete on a one-dimensional continuum of resources, e.g., food size. In a strictly unvarying (deterministic) environment, there is in general no limit to the degree of overlap, short of complete congruence. However, in a fluctuating (stochastic) environment, the average food sizes for species adjacent on the resource spectrum must differ by an amount roughly equal to the standard deviation in the food size taken by either individual species. This mathematical result emerges in a nonobvious yet robust way for environmental fluctuations whose variance relative to their mean ranges from around 0.01% to around 30%. In short, there is an effective limit to niche overlap in the real world, and this limit is insensitive to the degree of environmental fluctuation, unless it be very severe. Recent field work, particularly on bird guilds, seems in harmony with the model's conclusion.
TL;DR: It appears that the parathyroid hormone serves as a tropin for production of 1,25-dihydroxycholecalciferol, the hormonal form of vitamin D responsible for calcium mobilization from intestinal contents and bone.
Abstract: Thyroparathyroidectomy of rats on a diet low in calcium reduces production of 1,25-dihydroxycholecalciferol from 25-hydroxycholecalciferol to negligible levels within 40 hr, and increases production of another metabolite, called Va Parathyroid extract, at a dose of 20 units per day, prevents these changes When 40 units per day of parathyroid extract is given 48 hr after thyroparathyroidectomy, 1,25-dihydroxycholecalciferol production is restored almost to control levels within 36 hr The change brought about by parathyroid extract cannot be attributed to resulting changes in serum calcium or phosphorus concentration It appears that the parathyroid hormone serves as a tropin for production of 1,25-dihydroxycholecalciferol, the hormonal form of vitamin D responsible for calcium mobilization from intestinal contents and bone
TL;DR: Estimates of exponential relaxation times, t(r), for avifaunas of New Guinea satellite islands are calculated from analysis of four "experiments of nature": recolonization of exploded volcanoes, contraction in island area due to rising sea level, severing of land bridges, and disappearance of landbridge relict species.
Abstract: When species diversity S on an island is displaced from the equilibrium value by injection or removal of species, S relaxes to equilibrium by an imbalance between immigration and extinction rates. Estimates of exponential relaxation times, tr, for avifaunas of New Guinea satellite islands are calculated from analysis of four “experiments of nature”: recolonization of exploded volcanoes, contraction in island area due to rising sea level, severing of land bridges, and disappearance of landbridge relict species. tr is in the range 3,000-18,000 years for avifaunas of islands of 50-3000 square miles (130-7800 km2), and increases with island area. Immigration coefficients decrease and extinction coefficients increase with increasing S. The results may be relevant to the design of rainforest preserves.
TL;DR: Hybridization of (3)H-labeled ribosomal RNA to human chromosomes on slides resulted in specific labeling of the satellite regions of chromosomes 13, 14, 15, 21, and 22, with an over-all efficiency of about 5%.
Abstract: Hybridization of 3H-labeled ribosomal RNA to human chromosomes on slides resulted in specific labeling of the satellite regions of chromosomes 13, 14, 15, 21, and 22, with an over-all efficiency of about 5%. Differences between D and G chromosomes, and between associated and unassociated satellites, were not significant. Labeling of all other parts of the preparations was nonspecific, and increased in the order: extrachromosomal regions < chromosome arms < centric regions.
TL;DR: The tentative amino-acid sequence and three-dimensional structure of the lectin concanavalin A have been determined and the point of cleavage for the formation of the naturally occurring fragments A(1) and A(2), have been tentatively assigned.
Abstract: The tentative amino-acid sequence and three-dimensional structure of the lectin concanavalin A have been determined. The amino-acid sequence, which was determined chemically, contains 238 residues. The sequences of three short stretches were assigned on the basis of x-ray crystallographic data. Interpretation of an electron density map at 2-A resolution indicates that the predominant structural element is extended polypeptide chain arranged in two anti-parallel pleated sheets or β-structures. Residues not included in the β-structures are arranged in regions of random coil. One of the pleated sheets contributes extensively to the interactions among the monomers to form both dimers and tetramers. The positions at which Mn2+, Ca2+, and saccharide are bound to the protein, and the point of cleavage for the formation of the naturally occurring fragments A1 and A2, have been tentatively assigned. Both metal-binding sites are at least 20-A removed from the position at which saccharides are bound. The saccharide-binding site is a deep pocket of approximately 6A × 7.5A × 18A, the inner portion of which is occupied by hydrophobic residues.
TL;DR: X-chromosome inactivation during spermatogenesis is proposed as the ideal system for studies of genetic control at the chromosomal level.
Abstract: Inactivation of the single X chromosome in the primary spermatocytes of species with heterogametic males is postulated as a basic control mechanism on the chromosomal level that is required for normal spermatogenesis. This view is supported by (a) cytological observations of X-chromosome allocycly in the primary spermatocytes of all male-heterogametic organisms that were adequately examined, (b) autoradiographic evidence of early cessation of transcription by the X chromosome in the mouse and three species of grasshopper, and (c) the male sterility of animals with certain X-chromosome rearrangements that cannot be attributed to misfunction of specific genes. X-chromosome inactivation during spermatogenesis is proposed as the ideal system for studies of genetic control at the chromosomal level.
TL;DR: It is concluded that the majority of "intrinsic" membrane proteins have low polarity, and that the polarity index is therefore a useful parameter for characterization of membrane proteins.
Abstract: The polarities of a large number of soluble and membrane proteins have been calculated by summing the mole fractions of polar amino acids. It was found that 85% of the 205 soluble proteins considered in this study had polarities of 47 ± 6%. Only 2% of the soluble proteins had polarities below 40%, whereas 47% of the 19 membrane proteins had polarities below 40%. The membrane proteins with polarities below 40% could be separated from their respective membranes only by detergents or organic solvents, indicating the importance of hydrophobic forces in their interaction with other membrane components. It is concluded that the majority of “intrinsic” membrane proteins have low polarity, and that the polarity index is therefore a useful parameter for characterization of membrane proteins.
TL;DR: The primary structure of ovine hypothalamic hypophysiotropic luteinizing hormone-releasing factor, LRF, has been established as pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gel-NH(2) by hydrolysis of the peptide with chymotrypsin or pyrrolidone-carboxylylpeptidase and by analysis of the products by
Abstract: The primary structure of ovine hypothalamic hypophysiotropic luteinizing hormone-releasing factor, LRF, has been established as pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 by hydrolysis of the peptide with chymotrypsin or pyrrolidone-carboxylylpeptidase and by analysis of the products by an Edman-dansylation sequencing technique, as well as by mass spectrometry of the derived phenylthiohydantoins. A decapeptide with the proposed primary structure, prepared by total synthesis, gave the same result on sequencing. The synthetic decapeptide possesses the same biological activities as the native ovine LRF. The amino-acid sequence of ovine LRF is identical to that already published for porcine LRF.
TL;DR: Blood leukocytes of two species of new world primates, other than human, transform following exposure to Epstein-Barr virus, and reveal intranuclear inclusions; in both species, a large proportion of giant cells contain Epstein-Bs virus antigen detectable by immunofluorescence.
Abstract: Blood leukocytes of two species of new world primates, other than human, transform following exposure to Epstein-Barr virus. The transformed simian cells produce Epstein-Barr virus antigens and infectious (transforming) virus. The simian lymphoblastoid cells form multinucleate giant cells that appear to be selective sites for the production of Epstein-Barr virus. Multinucleate cells reveal intranuclear inclusions; in both species, a large proportion of giant cells contain Epstein-Barr virus antigen detectable by immunofluorescence.
TL;DR: Improvements in the method for localization of dopamine-beta-hydroxylase by immunofluorescence allow the observation of noradrenergic-cell bodies, non-terminal fibers, and axon terminals in the rat brain, and these observations suggest that cerebral microcirculation is regulated by central nor adrenergic neurons.
Abstract: Improvements in the method for localization of dopamine-β-hydroxylase by immunofluorescence allow the observation of noradrenergic-cell bodies, non-terminal fibers, and axon terminals in the rat brain. The distribution of the hydroxylase correlated well with the results obtained by localization of norepinephrine. Dopamine-β-hydroxylase was not observed in dopaminergic neurons or terminals, indicating that these cells do not have the capacity to synthesize norepinephrine. The use of the hydroxylase as a marker, however, has made it possible to visualize noradrenergic nerve terminals on small arteries in the brain parenchyma that have not been described by catecholamine-fluorescence histochemistry. The source of the terminals on small arteries appears to be central noradrenergic neurons rather than the peripheral sympathetic nervous system. Dopamine-β-hydroxylase generally was not observed in the large arteries of the brain parenchyma. These observations suggest that cerebral microcirculation is regulated by central noradrenergic neurons.
TL;DR: Butterflies of the neotropical Genus Heliconius feed on pollen; this is the first known instance in butterflies of a habit that is well known for other insects.
Abstract: Butterflies of the neotropical Genus Heliconius feed on pollen. This is the first known instance in butterflies of a habit that is well known for other insects. The butterflies remove amino acids and proteins from pollen; this feeding innovation plays a role in the reproductive and population biology of these insects. It is suggested that other animals may use pollen in a similar fashion.
TL;DR: Findings suggest that the glycoprotein is oriented at the cell surface with its oligosaccharide-rich N-terminal end exposed to the exterior, while its C- terminal segment interacts with other components in the interior of the membrane to form intramembranous particles.
Abstract: The major glycoprotein of the human erythrocyte membrane has been isolated by treatment with lithium di-iodosalicylate and found to be a single polypeptide chain with a molecular weight of about 50,000. This molecule, which is 60% carbohydrate and 40% protein, carries multiple blood-group antigens, the receptors for influenza viruses, and various plant agglutinins. Four unique carbohydrate-containing peptides (α-1, α-2, α-3, and β) are produced by tryptic digestion of the isolated glycoprotein; their order in the molecule has been determined by sequential tryptic digestion of intact erythrocyte membranes and partially digested glycoprotein fragments. Cleavage of the native protein with cyanogen bromide produces five fragments; two of these (C-5 and C-1) contain most of the carbohydrate in the molecule and are derived from the N-terminal half of the polypeptide chain. The nonpolar amino acids of this glycoprotein are located predominantly in the C-terminal fragment (C-2).
Phytohemagglutinin conjugated to ferritin has been used to map the distribution of glycoprotein receptors over the surfaces of intact erythrocytes by freeze-etching and electron microscopy. This label localizes to sites on the membrane that overlie the intramembranous particles. These findings suggest that the glycoprotein is oriented at the cell surface with its oligosaccharide-rich N-terminal end exposed to the exterior, while its C-terminal segment interacts with other components in the interior of the membrane to form intramembranous particles.
TL;DR: The quantitative features of the detergent extractions and the specific insulin-binding properties of the material so obtained indicate that the protein solubilized is the biologically significant insulin receptor, whose insulin- binding function is essentially unaltered.
Abstract: Extraction of liver and fat-cell membranes with the nonionic detergent Triton X-100 prevents specific binding of 125I-labeled insulin to these membranes. This loss of binding to particulate material is quantitatively recovered in a high-speed (300,000 × g, 2 hr) supernatant of the extract. Specific and reversible insulin binding to soluble proteins is readily demonstrable by gel filtration. A simple and sensitive assay for detection of specific macromolecule-insulin complexes has been developed based on the selective precipitation of the complex by polyethylene glycol. Extraction of membrane lipids with organic solvents or by phospholipase digestion does not impair the subsequent extraction of the insulin-binding protein with detergent. Binding of insulin to the soluble protein is a saturable and dissociable process having a dissociation constant of about 100 nM. Derivatives of insulin compete for binding in direct proportion to their biological activity; other peptide hormones are without effect. The quantitative features of the detergent extractions and the specific insulin-binding properties of the material so obtained indicate that the protein solubilized is the biologically significant insulin receptor, whose insulin-binding function is essentially unaltered.
TL;DR: It is demonstrated that the nonspecific crossreacting antigen is not a fragment of the carcinoembryonic antigen molecule and indicates common determinants on the molecules of both antigens.
Abstract: A glycoprotein present in normal human tissue is characterized that is neither organ- nor tumor-specific (nonspecific crossreacting antigen) and that crossreacts (by the Ouchterlony double-diffusion technique) with the carcinoembryonic antigen. This immunological relationship indicates common determinants on the molecules of both antigens. We demonstrate that the nonspecific crossreacting antigen is not a fragment of the carcinoembryonic antigen molecule.
TL;DR: The biochemical and morphological characteristics of the somatically produced hybrid were identical to those of the sexually produced amphiploid.
Abstract: Interspecific plant hybrids have been produced by parasexual procedures. Protoplasts of Nicotiana glauca and N. langsdorffii were isolated, fused, and induced to regenerate into plants. The somatic hybrids were recovered from a mixed population of parental and fused protoplasts by a selective screening method that relies on differential growth of the hybrid on defined culture media. The biochemical and morphological characteristics of the somatically produced hybrid were identical to those of the sexually produced amphiploid.
TL;DR: Findings suggest that beta(2)-microglobulin is a free immunoglobulin domain, possibly serving an effector function similar to that of the C(H)3 domain of gamma1 chains of immunoglOBulin G.
Abstract: Analysis of the primary structure of beta(2)-microglobulin indicates that this human protein is homologous in sequence to the constant portion of immunoglobulin light chains (C(L)), and to the homology regions (C(H)1, C(H)2, and C(H)3) of the constant portion of gamma1 (heavy) chains of immunoglobulin G. Homology with the C(H)3 region is particularly striking. No convincing homology could be demonstrated by similar comparisons with the variable regions of immunoglobulin light and heavy chains. beta(2)-Microglobulin contains an intrachain disulfide loop of 57 amino-acid residues that is similar in size to disulfide loops found in the constant regions of immunoglobulin G. These findings suggest that beta(2)-microglobulin is a free immunoglobulin domain, possibly serving an effector function similar to that of the C(H)3 domain of gamma1 chains of immunoglobulin G.
TL;DR: A revised hypothesis is presented in which a temporally discrete rise in lymphocyte cyclic GMP concentration is presented as an active signal to induce proliferation, while the elevation of cyclic AMP concentration in these cells is viewed as a regulatory influence that limits or inhibits mitogenic action.
Abstract: Adenosine 3′:5′-cyclic monophosphate (cyclic AMP) has been shown to have an antimitotic role in various cell types, and it has been hypothesized that a decrease of cyclic AMP concentration in the cell initiates or permits cell division. This hypothesis has been evaluated with respect to clonal proliferation of lymphocytes. Two potent mitogenic agents, phytohemagglutinin and concanavalin A, which induce thymic-dependent lymphocytes to undergo clonal proliferation, were examined for their ability to initiate proliferation and to alter the concentrations of cyclic AMP and guanosine 3′:5′-cyclic monophosphate (cyclic GMP) in purified human peripheral blood-lymphocytes. Optimal mitogenic concentrations of phytohemagglutinin and concanavalin A produced 10- to 50-fold increases in the concentration of lymphocyte cyclic GMP within the first 20 min of exposure to the mitogens. No changes were seen in the concentration of cyclic AMP after stimulation with either mitogen in purified form. Increases of less than 2-fold in the concentration of lymphocyte cyclic AMP observed with a less purified preparation of phytohemagglutinin could be attributed to the agglutinating rather than the mitogenic properties of the mitogen. A revised hypothesis is presented in which a temporally discrete rise in lymphocyte cyclic GMP concentration is viewed as an active signal to induce proliferation, while the elevation of cyclic AMP concentration in these cells is viewed as a regulatory influence that limits or inhibits mitogenic action.
TL;DR: The utility of a set of bacterial strains for detecting carcinogens as mutagens is shown and these compounds may be frameshiftmutagens of the class that intercalates into DNA and then reacts covalently with the DNA; various ultimate carcinogens may be of this type.
Abstract: Several carcinogenic metabolites of the carcinogen 2-acetyl-aminofluorene, especially 2-nitrosofluorene and N-hydroxy-2-aminofluorene, are potent frameshift mutagens for Salmonella typhimurium. 2-Nitrosonaphthalene, 2-nitrosophenanthrene, 4-nitroso-trans-stilbene, 4-nitrosobiphenyl, and 4-nitrosoazobenzene, all of which are metabolites or likely metabolites of carcinogenic aromatic amines, are also potent frameshift mutagens. These compounds may be frameshift mutagens of the class that intercalates into DNA and then reacts covalently with the DNA; various ultimate carcinogens may be of this type. The utility of a set of bacterial strains for detecting carcinogens as mutagens is shown.
TL;DR: Emphasis is given to a previously ignored possibility in which circadian organization is involved in photoperiodism, but not as the clock responsible for the time-measurement.
Abstract: It is an established fact that circadian rhythmicity is often somehow involved in the physiology of photoperiodic induction. It is shown, however, that there are three possible ways in which such rhythmicity could be involved. For the most part available data are inadequate to discriminate among these three roles, only one of which is covered by “Bunning's Hypothesis.” Emphasis is given to a previously ignored possibility in which circadian organization is involved in photoperiodism“ut not as the clock responsible for the time-measurement. The meaning of circadian surfaces and their bearing on the interpretation of a widely used experimental protocol is developed.
TL;DR: Like the T4 gene-32 protein characterized previously, the E. coli DNA-unwinding protein depresses the melting temperature of double-stranded DNAs, with regions rich in A-T base-pairs being preferentially melted.
Abstract: A DNA-unwinding protein has been purified to homogeneity from E. coli. This protein has a molecular weight of about 22,000, as judged by its electrophoretic mobility on polyacrylamide gels containing sodium dodecylsulfate, and it appears to be present in about 800 copies per log-phase cell. It binds tightly and cooperatively to single-stranded DNA, and much less tightly, if at all, to RNA or double-stranded DNA.
Like the T4 gene-32 protein characterized previously, the E. coli DNA-unwinding protein depresses the melting temperature of double-stranded DNAs, with regions rich in A-T base-pairs being preferentially melted. The E. coli protein strongly stimulates in vitro DNA synthesis by E. coli DNA polymerase II on appropriate templates; however, no stimulation is found with purified polymerases I or III of E. coli, or with T4 DNA polymerase. In contrast, gene-32 protein stimulates only the T4 DNA polymerase in a parallel assay.