A model for interphase chromosomes and evaluation of radiation-induced aberrations

We have developed a theoretical model for evaluating radiation-induced chromosomal exchanges by explicitly taking into account interphase (G(0)/G(1)) chromosome structure, nuclear organization of chromosomes, the production of double-strand breaks (DSBs), and the subsequent rejoinings in a faithful or unfaithful manner. Each of the 46 chromosomes for human lymphocytes (40 chromosomes for mouse lymphocytes) is modeled as a random polymer inside a spherical volume. The chromosome spheres are packed randomly inside a spherical nucleus with an allowed overlap controlled by a parameter Omega. The rejoining of DSBs is determined by a Monte Carlo procedure using a Gaussian proximity function with an interaction range parameter sigma. Values of Omega and sigma have been found which yield calculated results of interchromosomal aberration frequencies that agree with a wide range of experimental data. Our preferred solution is one with an interaction range of 0.5 microm coupled with a relatively small overlap parameter of 0.675 microm, which more or less confirms previous estimates. We have used our model with these parameter values and with resolution or detectability limits to calculate yields of translocations and dicentrics for human lymphocytes exposed to low-LET radiation that agree with experiments in the dose range 0.09 to 4 Gy. Five different experimental data sets have been compared with the theoretical results. Essentially all of the experimental data fall between theoretical curves corresponding to resolution limits of 1 Mbp and 20 Mbp, which may reflect the fact that different investigators use different limits for sensitivity or detectability. Translocation yields for mouse lymphocytes have also been calculated and are in good agreement with experimental data from 1 cGy to 10 cGy. There is also good agreement with recent data on complex aberrations. Our model is expected to be applicable to both low- and high-LET radiation, and we include a sample prediction of the yield of interchromosomal rejoining in the dose range 0.22 Gy to 2 Gy of 1000 MeV/nucleon iron particles. This dose range corresponds to average particle traversals per nucleus ranging from 1.0 to 9.12.     

Spatial distribution and yield of DNA double-strand breaks induced by 3-7 MeV helium ions in human fibroblasts

Accelerated helium ions with mean energies at the target location of 3-7 MeV were used to simulate alpha-particle radiation from radon daughters. The experimental setup and calibration procedure allowed determination of the helium-ion energy distribution and dose in the nuclei of irradiated cells. Using this system, the induction of DNA double-strand breaks and their spatial distributions along DNA were studied in irradiated human fibroblasts. It was found that the apparent number of double-strand breaks as measured by a standard pulsed-field gel assay (FAR assay) decreased with increasing LET in the range 67-120 keV/microm (corresponding to the energy of 7-3 MeV). On the other hand, the generation of small and intermediate-size DNA fragments (0.1-100 kbp) increased with LET, indicating an increased intratrack long-range clustering of breaks. The fragment size distribution was measured in several size classes down to the smallest class of 0.1-2 kbp. When the clustering was taken into account, the actual number of DNA double-strand breaks (separated by at least 0.1 kbp) could be calculated and was found to be in the range 0.010-0.012 breaks/Mbp Gy(-1). This is two- to threefold higher than the apparent yield obtained by the FAR assay. The measured yield of double-strand breaks as a function of LET is compared with theoretical Monte Carlo calculations that simulate the track structure of energy depositions from helium ions as they interact with the 30-nm chromatin fiber. When the calculation is performed to include fragments larger than 0.1 kbp (to correspond to the experimental measurements), there is good agreement between experiment and theory.     

Sequence variations in small-subunit ribosomal RNAs of Hartmannella vermiformis and their phylogenetic implications

Evidence of associations between free-living amoebas and human disease has been increasing in recent years. Knowledge about phylogenetic relationships that may be important for the understanding of pathogenicity in the genera involved is very limited at present. Consequently, we have begun to study these relationships and report here on the phylogeny of Hartmannella vermiformis, a free-living amoeba that can harbor the etiologic agent of Legionnaires' disease. Our analysis is based on studies of small-subunit ribosomal RNA genes (srDNA). Nucleotide sequences were determined for nuclear srDNA from three strains of H. vermiformis isolated from the United Kingdom, Germany, and the United States. These sequences then were compared with a sequence previously obtained for a North American isolate by J. H. Gunderson and M. L. Sogin. The four genes are 1,840 bp long, with an average GC content of 49.6%. Sequence differences among the strains range are 0.38%-0.76%. Variation occurs at 19 positions and includes 2 single-base indels plus 14 monotypic and 3 ditypic single-base substitutions. Variation is limited to eight helix/loop structures according to a current model for srRNA secondary structure. Parsimony, distance, and bootstrap analyses used to examine phylogenetic relationships between the srDNA sequences of H. vermiformis and other eukaryotes indicated that Hartmannella sequences were most closely related to those of Acanthamoeba and the alga Cryptomonas. All ditypic sites were consistent with a separation between European and North American strains of Hartmannella, but results of other tests of this relationship were statistically inconclusive.      

Olfactory receptor cell activity under electrical polarization of the nasal mucosa in the frog. II. Responses to odour stimulation

The interactions between electrical polarizations of the olfactory epithelium and odour stimulations were investigated at the level of the extracellular spike activity of the receptor cells in the frog. 1. In most cases, surface positive polarizations enhanced the excitatory olfactory responses, negative polarizations suppressed these responses; both interactive effects were graded. 2. The response of receptor cells to electrical polarization was markedly reduced or suppressed for several seconds following olfactory stimulation. This effect and the time course of the recovery period depended on the nature and the concentration of the olfactory stimulus. 3. The decrease in electrical excitability seemed to be independent of whether the recorded neuron had responded or not to the prior olfactory stimulation. 4. It is suggested that the olfactory stimulation caused the total constant current to change its distribution in the different cell pathways. Changes in conductance induced by olfactory stimuli could implicate the supporting cells. 5. The experimental findings are discussed with reference to a model of receptor cell function that assumes a deep, axo-somatic localization of the action potential trigger-zone.     

Epidermal growth factor and the control of proliferation of Balb 3T3 and benzo[a]pyrene-transformed Balb 3T3 cells.

Benzo[a]pyrene-transformed Balb 3T3 cells (BP3T3) exhibit "normal" growth controls at low concentrations of serum. Epidermal growth factor (EGF) stimulates DNA synthesis and cell division in both Balb 3T3 and BP3T3 cells at physiological concentrations. The growth response of BP3T3 cells to EGF is qualitatively the same as that of 3T3 cells, however, the transformed cells have a lower quantitative requirement. Both 3T3 and BP3T3 cells show a density-dependent response to EGF, but the shift in the dose response curve for BP3T3 cells at high cell density is smaller than that seen for 3T3 cells. One cause of the restricted growth of 3T3 cells at high cell density compared with BP3T3 cells is the increased concentration of growth factor needed for stimulation of 3T3 cells at higher cell densities. A lower rate of depletion of other growth factory by BP3T3 cells may also explain the smaller effect of cell density on the EGF response of these cells.     

Cell-to-cell transfer of glial proteins to the squid giant axon: The glia- neuron protein transfer hypothesis

The hypothesis that glial cells synthesize proteins which are transferred to adjacent neurons was evaluated in the giant fiber of the squid (Loligo pealei). When giant fibers are separated from their neuron cell bodies and incubated in the presence of radioactive amino acids, labeled proteins appear in the glial cells and axoplasm. Labeled axonal proteins were detected by three methods: extrusion of the axoplasm from the giant fiber, autoradiography, and perfusion of the giant fiber. This protein synthesis is completely inhibited by puromycin but is not affected by chloramphenicol. The following evidence indicates that the labeled axonal proteins are not synthesized within the axon itself. (a) The axon does not contain a significant amount of ribosomes or ribosomal RNA. (b) Isolated axoplasm did not incorporate [(3)H]leucine into proteins. (c) Injection of Rnase into the giant axon did not reduce the appearance of newly synthesized proteins in the axoplasm of the giant fiber. These findings, coupled with other evidence, have led us to conclude that the adaxonal glial cells synthesize a class of proteins which are transferred to the giant axon. Analysis of the kinetics of this phenomenon indicates that some proteins are transferred to the axon within minutes of their synthesis in the glial cells. One or more of the steps in the transfer process appear to involve Ca++, since replacement of extracellular Ca++ by either Mg++ or Co++ significantly reduces the appearance of labeled proteins in the axon. A substantial fraction of newly synthesized glial proteins, possibly as much as 40 percent, are transferred to the giant axon. These proteins are heterogeneous and range in size from 12,000 to greater than 200,000 daltons. Comparisons of the amount of amino acid incorporation in glia cells and neuron cell bodies raise the possibility that the adaxonal glial cells may provide an important source of axonal proteins which is supplemental to that provided by axonal transport from the cell body. These findings are discussed with reference to a possible trophic effect of glia on neurons and metabolic cooperation between adaxonal glia and the axon.     

Bacteriophage P22 as a vector for Mu mutagenesis in Salmonella typhimurium: isolation of nad-lac and pnc-lac gene fusions.

Salmonella phage P22 was utilized as a vector for phage Mu cts d1(Apr lac) mutagenesis in Salmonella typhimurium. Efficient transposition of phage Mu d1 and the construction of gene fusions were readily accomplished with this procedure. Mutants blocked in the biosynthesis of NAD+ and in pyridine nucleotide cycle metabolism were isolated by this method, resulting in nadB-lac, nadC-lac, and pncB-lac gene fusions.     

Metal levels associated with tin dredging and smelting and their effect upon intertidal reef flats at ko phuket,Thailand

Summary Reef flats in the vicinity of tin dredging and smelting activities around the Laem Pan Wah peninsula, Phuket, and been quantitatively surveyed. the diversity of corals on all intertidal reefs was low (10 genera), the dominant genera being Porites, Montipora, Acropora and Platygyra. Two basic types of reef can be discerned, one dominated by Porites lutea and faviid species and the other by Montipora ramosa and Acropora aspera, reef type apparently being governed by the degree of exposure to water movement. Other natural factors affecting coral cover included freshwater run off, considerable sedimentation, and aerial exposure for 2–3 h each day. Heavy metal concentrations in invertebrate species such as the oyster Saccostrea, the bivalve Isognomon, and in the alga Padina reflected elevated metal levels at all sites when compared with controls (Figs. 8 and 9). In particular, levels of metals were considerably elevated in molluscs taken from the reef below the tin smelter. Interestingly, dead coral cover on this reef, although high, was not significantly different from values observed on reefs several kilometres away from the smelter, which were not apparently under the influence of such increased metal loads (Fig. 2).No elevation in metal concentrations in coral tissue or skeleton was evident at any site. It would appear, then, that these intertidal coral species are not obviously affected by the levels of metals discharged at the smelter site.