Vasoactive agents which elevate either cGMP or cAMP inhibit platelet activation by pathways sharing at least one component, the 46/50 kDa vasodilator-stimulated phosphoprotein (V ASP). V ASP is stoichiometrically phosphorylated by both cGMP-dependent and cAMPdependent protein kinases in intact human platelets, and its phosphorylation correlates very well with platelet inhibition caused by cGMP- and cAMP-elevating agents. Here we report that in human platelets spread on glass, V ASP is associated predominantly with the distal parts of radial micro filament bundles and with microfilaments outlining the periphery, whereas less V ASP is associated with a central microfilamentous ring. V ASP is also detectable in a variety of different cell types including fibroblasts and epithelial cells. In fibroblasts, V ASP is concentrated at focal contact areas, along microfilament bundles (stress fibres) in a punctate pattern, in the periphery of protruding lamellae, and is phosphorylated by cGMP- and cAMP-dependent protein kinases in response to appropriate stimuli. Evidence for the direct binding of V ASP to F -actin is also presented. The data demonstrate that V ASP is a novel phosphoprotein associated with actin filaments and focal contact areas, i.e. transmembrane junctions between microfilaments and the extracellular matrix.

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Electron microscopic spread preparations of oocyte nucleoli (lampbrush stage) of various amphibians are quantitatively evaluated and the length distributions of repeat-, matrix-, and spacer-units along the rRNA cistron containing axes are given. The correlation of the matrix unit data with the gel electrophoretic pattern of labelled nuclear RNA from the same oocytes is examined. The mean value of the matrix unit corresponds fairly well to a 2.6 million D peak of pre-rRNA but the distribution of both matrix units and labelled pre-rRNAs shows an asymmetrical heterogeneity indicating the existence of some larger primary transcription products of rDNA. Novel structural aspects are described in the spacer regions which suggest that transcription does also take place in DNP regions between the matrix units. A special "prelude piece" coding for approx. 0.5 million D of RNA is frequently visualized in the spacer segments at the beginning of a matrix unit. Possible artifacts resulting from the preparation, the relative congruence between the data obtained using both methods, and the functional meaning of the findings are discussed against the background of current concepts of structural organization and transcription products of nucleolar DNA.

The arrangement of transcriptional units in the loops of lampbrush chromosomes from oocyte nuclei of urodele amphibia and from primary nuclei of the green alga Acetabularia have been studied in the electron microscope using spread preparations. Loops with different patterns of arrangement of matrix units (i.e. to a first approximation, transcriptional units) can be distinguished: (i) loops consisting of one active transcriptional unit; (ii) loops containing one active transcriptional unit plus additional fibril-free, i.e. apparently untranscribed, intercepts that may include 'spacer' regions; (iii) loops containing two or more transcriptional units arranged in identical or changing polarities, with or without interspersed apparent spacer regions. Morphological details of the transcriptional complexes are described. The observations are not compatible with the concept that one loop reflects one and only one transcriptional unit but, rather, lead to a classification of loop types according to the arrangement of their transcriptional units. We propose that the lampbrush chromosome loop can represent a unit for the coordinate transcription of either one gene or a set of several (different) genes.

No abstract available