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G Rasigni

Publications and source records attributed to G Rasigni.

7 recordsLinked to original sources
PubMed1990

Abortive intermediates in transcription by wheat-germ RNA polymerase II. Dynamic aspects of enzyme/template interactions in selection of the enzyme synthetic mode.

At constant enzyme concentration and with the full set of nucleotide substrates dictated by template sequence, the chain-length distribution of polymeric product varies with template concentration in reactions catalysed by wheat-germ RNA polymerase II. Under the same conditions, but in the presence of a single ribonucleoside triphosphate, the rate of condensation of the triphosphate substrate to a dinucleotide primer also exhibits a complex dependence with the template concentration. This effect is observed using poly[d(A-T)] as a template. For both reactions there are two extreme types of behaviour in each of which transcription appears to involve a single enzyme synthetic mode, characterized by either a high (at low template concentration) or a low (at high template concentration) probability of releasing the transcripts. A strong correlation is found between these two pathways, such that conditions favouring the abortive release of trinucleotide products in the single-step addition reaction are associated with the synthesis of short-length RNA species in productive elongation, and reciprocally. A model previously developed by Papanicolaou, Lecomte & Ninio [(1986) J. Mol. Biol. 189, 435-448] to account for the kinetics of polymerization/excision ratios with Escherichia coli DNA polymerase I, and by Job, Soulié, Job & Shire [(1988) J. Theor. Biol. 134, 273-289] for kinetics of RNA-chain elongation by wheat-germ RNA polymerase II provides an explanation for the observed behaviour with the plant transcriptase. The basic requirement of this model is a slow equilibrium between two states of the polymerization complex with distinct probabilities of releasing the product. In the presence of Mn2+, and under conditions allowing the synthesis of poly[r(A-U)], one of these states is involved in the formation of oligonucleotides shorter than 15 bases, whereas the other catalyses the polymerization of chains longer than 40 bases.

DNA↗
PubMed1989

Molecular organization and clustering of cell-wall-bound enzymes as a source of kinetic apparent co-operativity.

When fixed charges and enzyme molecules are not homogeneously distributed in a matrix, the degree of organization of charges, of enzyme molecules and of charges with respect to enzyme molecules modulate the enzyme reaction rate. The overall reaction velocity of the bound enzyme system may be expressed in terms of monovariate moments of the charge density distribution and of the bivariate moments of the charge and enzyme density distributions. With respect to the situation where fixed charges and enzyme molecules are randomly distributed in the matrix, the molecular organization, as expressed by the monovariate and bivariate moments results in an increase or a decrease, of the overall reaction rate, as well as in the appearance of a kinetic cooperativity. The degree of spatial organization of objects may be expressed quantitatively through the concept of minimal spanning tree. This concept may thus be applied to the quantification of the degree of order that may exist in the bidimensional distribution of enzyme molecules in a charged matrix. Primary walls of isolated plant cells in sterile culture behave as a polyanion and contain different enzymes. The spatial distribution in sycamore cell walls of an acid phosphatase has been studied through the concept of minimal spanning tree and shown to be non-randomly distributed in the polyanionic matrix, but clustered in that matrix. This spatial organization results in a modulation of the reaction rate of the cell-wall-bound phosphatase reaction. Both the theoretical and experimental results presented in this study leave little doubt as to the validity of the idea that in situ the organization of fixed charges and enzyme molecules modulate the overall dynamics of enzyme reactions.

Acid Phosphatase↗
PubMed1989

Toward a new approach in tumor cell heterogeneity studies using the concept of order.

A new methodology was developed to study dynamic processes topographically in biological systems by means of a graph-theoretical method. It is based upon order parameters obtained from a minimal spanning tree analysis coupled with computer simulations. The method was used to analyse the heterogeneous behavior of two neoplastic cell lines after treatment with laminin. The laminin-induced cell detachment was quantitated and shown to be inversely related to cell population density and thus to cellular interactions. Our statistical analysis is a very powerful tool to obtain information from seemingly disorderly heterogeneous biological models.

Animals↗
PubMed1988

Quantization of directional properties in biological structures using the Minimal Spanning Tree.

A method that uses the Minimal Spanning Tree graph has previously been developed (Dussert et al., 1987, J. theor. Biol. 125, 317) in order to analyse the degree of order in biological structures. This graph is shown here to be very powerful in bringing out directional properties of biological structures which cannot be revealed by a simple visual examination. The method is illustrated by means of various computer simulations.

Computer Simulation↗
PubMed1987

Minimal spanning tree analysis of biological structures.

A new approach to study order and disorder in biological membranes and more generally in biological structures is developed. It is based on a graph constructed on the set points representing the position of particles. From this graph, which is called the minimal spanning tree, it is possible to deduce two parameters, namely the average length m and the standard deviation sigma which are characteristic of the repartition to be studied. The use of a diagram involving both m and sigma makes it possible to determine the degree of order by taking a simple reading in the (m, sigma) plane.

Cell Membrane↗
PubMed1985

Quantitative characterization of a biological membrane by means of its spatial autocovariance.

Profiles for the exoplasmic face (EF) of the freeze-fractured plasma membrane from the root storage tissue of red beets are reconstructed by microdensitometry of micrographs of surface-shadowed-platinum carbon replicas. Autocovariance functions (ACFs) are computed from those profiles. The initial portions of the ACFs have a Gaussian form whose parameters (root mean square surface roughness and autocovariance length) are estimated. The parameter estimates are used to show that the pits on the EF faces are in good complementarity with the intramembrane particles seen on the complementary protoplasmic fracture faces.

Cell Membrane↗