Energy migration in the poly(ra-rU)-ethidium complex. determination of the unwinding angle of the polyribonucleic helix

The polarized fluorescence of the ethidium bromide (EB)-poly(rA-rU) complex has been studied by pulse fluorometry. As expected for a polynucleotide snowing one single kind of intercalation site, the decay of the whole emission is a single exponential (time constant 27 ns). The anisotropy decay is analysed as follows: (1) A brownian contribution having two correlation times, one of which characterizes local motions and the other a macromolecular motion. (2) A contribution due to transfers between EB molecules fixed to the same polynucleotide molecule, is analysed by a method analogous to the method used in previous work on EB-DNA complexes. That method consists in choosing a molecular model of the complex depending on geometrical parameters, and in simulating the energy migration on that model with a Monte Carlo calculation. Poly(rA-rU) is assumed here to adopt the structure A of RNA. Intercalated EB molecules modify the anale between two consecutive base pairs by δ. The angular position of the EB transition moment is defined by an angle φ. One finds that the angle φ is situated between 0° and 30°, which corresponds to a whole intercalation of the chroniophore as opposed to the semi-intercalation which has been proposed for certain dyes. The angle δ is negative; therefore there is an unwinding of the polyribonucleotide helix. Its absolute value is about 38°, sensibly greater than The value previously found for EB-DNA complexes.     

The use of fluorescence anisotropy decay of poly d(A-T) ethidium bromide complex to estimate the unwinding angle of the double helix.

We measured the fluorescence decay under polarized light, of ethidium bromide bound to the poly d(A-T) isolated from Cancer Pagurus. The decay of the whole fluorescence is a single exponential function revealing a good homogeneity of the binding sites. The anisotropy decay due to energy transfers between the ethidium bromide molecules bound to a same poly d(A-T) molecule has been analysed, with a Monte Carlo calculation. We found the dye unwinds the poly d(A-T) duplex by an angle of 17 degrees plus or minus 2 degrees. This result is in agreement with the value previously found in the case of calf thymus DNA-ethidium bromide complex, although the base compositions of the two nucleic acids are different.     

Heterogeneite des variations spectrales photoinduites vers 700 nm observees sur les membranes chlorophylliennes et les complexes chlorophylle-proteines isoles de divers organismes photosynthetiques

A heterogeneity of photoinduced spectral variations near 700 nm is observed at −196°C, by comparison of difference spectra obtained from several materials (two blue-green algae, Spirulina and Fremyella; a red alga Porphyridium and a higher plant, Nicotiana tabacum). The maximum of complexity is seen with Spirulina chlorophyllous membranes where three bands are detected at 718, 705 and 695 nm. These bands are recovered in relatively different amounts in two chlorophyll-protein complexes prepared after sodium dodecyl sulfate electrophoresis of Spirulina membranes.

The photoinduced changes near 700 nm with Spirulina membranes are reversible and seem to be synchronous and related to the bleaching of three different pigments. This spectral diversity for photosensizable pigments is comparable with the spectral heterogeneity of the major forms of the bulk chlorophyll. It is assumed that these pigments may play the same functional role. This assumption may be related to a possible heterogeneity of Photosystem I centres recently detected by Breton using linear dichroism method (Breton, J. (1977) Biochim. Biophys. Acta 459, 66–70).     

Perinuclear organelles of the sensory cell of the Grandry cutaneous corpuscle. Ultrastructure and formation

Perinuclear organelles are found in the sensory cell of the Grandry cutaneous corpuscle in the duck. They are ovoid, fusiform or conical. They measure up to 5 µ length and l µ in diameter. They are formed by regular alternation of granular layers (mornolayers of RNA-rich granules which can be interpreted as ribosomes) and fibrous layers (generally formed by two sublayers of parallel fibrils). These fibrils (60-80 Å diameter) are in continuity with the intra-cytoplasmic fibrils which are very abundant in the Grandry cell. The central part of the organelles is devoid of RNA-rich granules.The formation of these organelles begins about one week after hatching, in the cytoplasmic perinuclear area where abundant granular endoplasmic reticulum and very numerous ribosomes and fibrils are present. The function of these perinuclear organelles remains unknown.     

The hierarchical approach to the DNA stability problem. II. Some applications and speculations with yeast mitochondrial DNA as an example

As discussed in the preceding article [1] hierarchical analysis of DNA sequences should make it possible to treat complex unfolding (and refolding) processes involving both equilibrium and non-equilibrium subtransitions. Hence a variety of actual experimental situations may be analyzed. This is demonstrated with the help of a 1950 bp yeast mitochondrial DNA sequence encompassing part of the 21S ribosomal RNA gene: excellent fit of complex denaturation and renaturation profiles is achieved with only two adjustable parameters. The advantage of dealing with objectively defined stability units is also apparent when stability profiles are compared to known functional maps: striking correlations may be brought out and their possible significance is briefly discussed.     

A simple framework to describe the regulation of gene expression in prokaryotes

Based on the bimolecular mass action law and the derived mass conservation laws, we propose a mathematical framework in order to describe the regulation of gene expression in prokaryotes. It is shown that the derived models have all the qualitative properties of the activation and inhibition regulatory mechanisms observed in experiments. The basic construction considers genes as templates for protein production, where regulation processes result from activators or repressors connecting to DNA binding sites. All the parameters in the models have a straightforward biological meaning. After describing the general properties of the basic mechanisms of positive and negative gene regulation, we apply this framework to the self-regulation of the trp operon and to the genetic switch involved in the regulation of the lac operon. One of the consequences of this approach is the existence of conserved quantities depending on the initial conditions that tune bifurcations of fixed points. This leads naturally to a simple explanation of threshold effects as observed in some experiments.