Chromatin structure during the prereplicative phases in the life cycle of mammalian cells

The object of this study was to determine the kinetics of chromosome decondensation during the G₁ period of the HeLa cell cycle. HeLa cells synchronized in the G₁ period following the reversal of mitotic block were fused with Colcemid-arrested mitotic HeLa cells at 1.5, 3, 5, and 7 h after the reversal of N₂O block. The resulting prematurely condensed chromosomes (PCC) were classified into six categories depending on the degree of their condensation. The frequency of occurrence of each category was plotted as a function of time after mitosis. The results of this study indicate that the process of chromosome decondensation, initiated during the telophase of mitosis continues throughout the G₁ period without any interruption, thus the chromatin reaches an ultimate state of decondensation by the end of G₁ period, when DNA synthesis is initiated.     

Fusion of Avena sativa mesophyll cell protoplasts by electrical breakdown

Studies with the light microscope were carried out on mesophyll cell protoplasts of Avena sativa which had been made to undergo fusion by reversible electrical breakdown of the cell membrane. In order to establish close membrane contact between the cells, an important prerequisite for fusion, a method known as dielectrophoresis was used. In an inhomogeneous alternating electrical field the protoplasts adhere to the electrodes and to each other in the direction of the field lines. The cells which were thus brought into close contact with each other could be made to fuse by the application of a field pulse of high amplitude (about 750 V/cm) and short duration (20–50 μs). The field strength required for fusion exceeds the value necessary for the electrical breakdown of the cell membrane. Fusion took place within some minutes and led to a high yield of fused protoplasts. The fusion of cells being in the electric field occured in a synchronous manner. In some of the fusion experiments part of the protoplasts of A. sativa were stained with neutral red. When these cells were fused with unstained protoplasts, the vacuoles from the different cells within the fused aggregate could be shown to remain separate for quite some time.     

Yeast mitochondrial and cytoplasmic valyl-tRNA synthetases

Yeast mitochondrial tRNA synthetase has been partially purified and chromatographic, catalytic and antigenic properties have been compared to the cytoplasmic homologous enzyme from yeast. No significant differences could be observed between the two enzymes with respect to their behaviour during ammonium sulfate precipitation or in chromatographic separation on DEAE cellulose, hydroxylapatite and Sephadex G 200. The Km of the two enzymes toward tRNAs from yeast mitochondria, yeast cytoplasm or E. coli are pratically identical. The antigenic properties of the two enzymes are very similar; antisera against either the mitochondria or the cytoplasmic enzyme lead to the inhibition of their catalytic properties. The mitochondrial ValRS is formed by a single polypeptide chain whose molecular weight is 125,000 daltons, a value very close to that of the yeast cytoplasmic enzyme.     

Control of activity states of heart mitochondrial ATPase. Role of the proton-motive force and Ca2+

The ATPase complex of submitochondrial particles exhibits activity transitions that are controlled by the natural ATPase inhibitor (Gómez-Puyou, A., Tuena de Gómez-Puyou, M. and Ernster, L. (1979) Biochim. Biophys. Acta 547, 252–257). The ATPase of intact heart mitochondria also shows reversible activity transitions; the activation reaction is induced by the establishment of electrochemical gradients, whilst the inactivation reaction is driven by collapse of the gradient. In addition it has been observed that the influx of Ca²⁺ into the mitochondria induces a rapid inactivation of the ATPase; this could be due to the transient collapse of the membrane potential in addition to a favorable effect of Ca²⁺-ATP on the association of the ATPase inhibitor peptide to F₁-ATPase. This action of Ca²⁺ may explain why mitochondria utilize respiratory energy for the transport of Ca²⁺ in preference to phosphorylation. It is concluded that the mitochondrial ATPase inhibitor protein may exert a fundamental regulatory function in the utilization of electrochemical gradients.     

Binding of ADP to beef-heart mitochondrial ATPase (F1)

1. ADP binding to beef-heart mitochondrial ATPase (F₁), in the absence of Mg²⁺, has been determined by separating the free ligand by ultrafiltration and determining it in the filtrate by a specially modified isotachophoretic procedure.2. Since during the binding experiments the ‘tightly’ bound ADP (but not the ATP) dissociates, it is necessary to take this into account in calculating the binding parameters.3. The binding data show that only one tight binding site (K_d about 0.5 μM) for ADP is present.4. It is not possible to calculate from the binding data alone the number of or the dissociation constants for the weak binding sites. It can be concluded, however, that the latter is not less than about 50 μM.     

Somatic hybrids between unilateral cross-incompatible Petunia species

Summary Somatic hybrid plants regenerated following the fusion of leaf mesophyll protoplasts of Petunia parodii with those isolated from a cell suspension of albino P. inflata. These two species exhibit a unilateral cross-incompatability with a pre-zygotic mode of reproductive isolation preventing hybridizations with P. inflata as the maternal parent. Selection of somatic hybrids relied on the fact that unfused or homokaryon protoplasts of P. parodii did not develop beyond the cell colony stage while those of the putative somatic hybrids and albino P. inflata parent produced callus. Green somatic hybrid calluses were readily identified against the white background of P. inflata following complementation to chlorophyll synthesis proficiency and continued growth in hybrid cells. Shoots, and ultimately flowering plants, were identified as somatic hybrids based on their floral morphology and colour, chromosome number and the fact that they segregated for parental characters. The frequency of somatic hybrid production was comparable to that previously established for two sexually compatible Petunia species.     

Blue adaptation: An experimental tool for the study of visual receptor mechanisms and behaviour of Drosophila

Physiological and behavioural studies with Drosophila to elucidate visual mechanisms have exploited the bi-stability of the visual pigment in the peripheral retinula cells R1–6, and the off-on switch action of blue and orange light. Measurements of flicker fusion and response waveform from both receptor and lamina regions prior and subsequent to blue adaptation, which induces a prolonged depolarising afterpotential and loss of visual function in R1–6, show these retinula cells to have a high fusion frequency and R7/8, the central retinula cells, a lower fusion frequency. Such measurements also allow analysis of the extracellular response in terms of contributing cells, and its potential for studying the fly's ability to respond to various potential visual cues such as a rotating plane of polarised light. Blue adapted flies fail to fixate normally a black stripe, confirming a role for R1–6 in orientation behaviour requiring a competent degree of acuity.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Recombinant DNA analysis indicates that the multiple chromosomes of maize mitochondria contain different sequences

Twenty-eight Bam H 1 restriction fragments were isolated from normal mitochondrial DNA of maize by recombinant DNA techniques to investigate the organization of the mitochondrial genome. Each cloned fragment was tested by molecular hybridization against a Bam digest of total mitochondrial DNA. Using Southern transfers, we identified the normal fragment of origin for d each clone. Twenty-three of the tested clones hybridized only to the fragment from which the clone was derived. In five cases, labeling of an additional band indicated some sequence repetition in the mitochondrial genome. Four clones from normal mitochondrial DNA were found which share sequences with the plasmid-like DNAs, S-1 and S-2, found in S male sterile cytoplasm. The total sequence complexity of the clones tested is 121×10⁶ d (daltons), which approximates two thirds of the total mitochondrial genome (estimated at 183×10⁶ d). Most fragments do not share homology with other fragments, and the total length of unique fragments exceeds that of the largest circular molecules observed. Therefore, the different size classes of circular molecules most likely represent genetically discrete chromosomes in a complex organelle genome. The variable abundance of different mitochondrial chromosomes is of special interest because it represents an unusual mechanism for the control of gene expression by regulation of gene copy number. This mechanism may play an important role in metabolism or biogenesis of mitochondria in the development of higher plants.