The chromosome fiber: Evidence for an ordered superstructure of nucleosomes

Chromosome fibers isolated from lymphocyte nuclei and prepared for electron microscopy by techniques designed to preserve their native structure have a distinctly knobby appearance, suggesting that DNA and protein are not distributed evenly along the fiber axis. Individual knobs (superbeads) are arranged in tandem and have an average diameter of about 200 Å. Mild nuclease digestion of isolated nuclei releases apparent monomer superbeads that are composed of nucleohistone particles with the properties of nucleosomes. The kinetics of digestion indicate that the superbead is a discrete structural unit containing, on the average, about eight nucleosomes.     

The myosin filament: V. Intermediate voltage electron microscopy and optical diffraction studies of the substructure

Using a 200 kV electron microscope (JEM 200 A), thick (up to 0.4 μm) crosssections of the myosin filaments of vertebrate striated muscle were studied. It was found that: (a) with increasing section thickness the cross-sectional profiles of the shaft of the filament were increasingly more triangular and in sections 0.4 μm thick each apex of the triangle was clearly blunted. This unique cross-sectional profile is predicted by the model proposed by Pepe (1966,1967) in which 12 parallel structural units are packed to form a triangular profile with a structural unit missing at each apex of the triangle. (b) With increasing section thickness the substructure of the myosin filament was enhanced, with the best substructure visible in sections 0.2 μm to 0.3 μm thick. This strongly supports parallel alignment of structural units in the shaft of the filament as proposed by Pepe (1966,1967). (c) The substructure spacing, determined by optical diffraction from electron micrographs of cross-sections of individual myosin filaments or groups of filaments is about 4 nm. (d) The different optical diffraction patterns observed from individual myosin filaments can be explained if the projection of each structural unit in the plane of the section has an elongated profile. With a substructure spacing of 4 nm an elongated cross-sectional profile could be produced by having two myosin molecules per structural unit. Models drawn with two myosin molecules per structural unit in the model proposed by Pepe (1966,1967) gave optical diffraction patterns similar to those observed from individual filaments. (e) The different optical diffraction patterns observed from individual myosin filaments can be explained if the elongated profiles in each structural unit are similarly oriented but with the orientation changing along the length of the filament. The change in orientation per unit length of the filament must be small enough to maintain an elongated profile for the projection of the structural unit in the plane of the sections 0.3 μm thick. All of these observations and conclusions strongly support the model for the myosin filament proposed by Pepe (1966,1967).     

COMPARISON OF ATTACHED DIATOM COMMUNITIES ON NATURAL AND ARTIFICIAL SUBSTRATES1

A study of attached diatom communities on artificial and natural substrates was conducted in Wheelwright Pond, New Hampshire, during 1975. There were differences in the species composition growing on artificial glass slides and natural substrates. The slides favored the accumulation of Achnanthes minutissima Kütz. and decreased the relative abundance of Eunotia incisa W. Sm. ex. Greg. and Cocconeis placentula v. euglypta (Ehr.) Cleve. Large growths of planktonic species were also noted on the slides. Compositional differences between slides positioned at 15–30 cm (upper) and 1 m (lower) from the surface of the water were minimal. A similar community composition of periphytic diatoms was found on five aquatic macrophytes.     

Acetylene Reduction by Soil Cores of Maize and Sorghum in Brazil

Nitrogenase activity was measured by the C₂H₂ reduction method in large soil cores (29 cm in diameter by 20 cm in depth) of maize (Zea mays) and sorghum (Sorghum vulgare). The activity was compared to that obtained by a method in which the roots were removed from the soil and assayed for nitrogenase activity after an overnight preincubation in 1% O₂. In a total of six experiments and 28 soil cores, the nitrogenase activity of the cores was an average of 14 times less than the activity of roots removed from the same cores and preincubated. Nitrogenase activity in the cores was very low and extrapolated to an average nitrogen fixation rate of 2.8 g of N/hectare per day. It was shown that inadequate gas exchange was not a reason for the lower activity in the soil cores, and the core method gave satisfactory results for nitrogenase activity of soybeans (Glycine max) and Paspalum notatum.     

Abnormal functional development of sympathetic nerve terminal-smooth muscle complex in neonatal spontaneously hypertensive rats

In neonatal and young adult SH and WK rats, maturation of a functional unit consisting of sympathetic nerve terminals and smooth muscle (levator palpebrae) was assessed $\text{in vivo}$ by measuring the contractile response to tyramine, an agent which releases endogenous norepinephrine from sympathetic nerve terminals. Responses in SH are comparable to WK at 5–6 days postnatally, smaller from the 8th through 19th day and larger in young adults (41–46 days old). Results indicate that functional maturation of the nerve terminal-smooth muscle complex is retarded in SH relative to WK during the 2nd and 3rd postnatal weeks. It is suggested that retardation in the neonatal SH rat is an expression of a genetic defect in the growth of the complex and that the enhanced response in young adult SH is a consequence of the neonatal abnormality.     

Windrichtung und Vegetationsdeckung am Nest der Silberm?we(Larus argentatus)

Zusammenfassung An 1047 Silbermöwen-Nestern auf den Watteninseln Mellum und Memmert wurden 1979/80 ermittelt (Abb. 1): Grenzrichtungen der Vegetationsstände 1–3 (1: Vegetationsstand größter Winkelgröße) und Nest-Zugänge 1–3, ihre Winkelgrößen und mittleren Richtungen. Mehr als 80% der Nester waren zu 40–90% von Vegetation umstanden (Abb. 2). Die Winkelgröße des Gesamtvegetationsstandes betrug auf Mellum 212°, auf Memmert 205° (Tab. 1). Die Nestzugänge hatten eine geringere Winkelgröße als die Vegetationsstände (Tab. 1, Abb. 3). Die Silbermöwe bevorzugte demnach Nestplätze, die von Vegetation schützend umgeben sind. Der Vegetationsstand 1 befand sich bevorzugt auf der westlichen (= Hauptwindrichtung), der Zugang 1 auf der östlichen Nestseite (Tab. 2, Abb. 4). Die Verteilung der Gesamtvegetation zeigte entsprechende Vorzugsrichtungen (Abb. 6). Im Vergleich zu 1979 war 1980 eine Verlagerung der Vorzugsrichtung des Vegetationsstandes 1 um 50–60° von West nach Nord festzustellen (Tab. 2, Abb. 4). Ein Vergleich der mittleren Windrichtungen beider Jahre zeigte eine gleichsinnige Richtungsänderung (Abb. 5). Auf den zwei untersuchten Probeflächen Mellums ließ sich eine Auswahl des Nestplatzes im Hinblick auf Sichtschutz gegen Nachbarn nicht nachweisen (Tab. 3). Die mögliche Funktion der nestumgebenden Vegetation als Wind- und Wetterschutz wird diskutiert.

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Vegetation surrounding Herring Gulls'(Larus argentatus) nests in relation to wind direction

Summary The following measurements were taken for a total of 1047 Herring Gulls' nests on the North Sea islands Mellum and Memmert (West Germany) in 1979 and 1980 (cf. Fig. 1 for terminology): limiting directions, angular sizes, and mean directions of the surrounding vegetation-stands 1 to 3 (1 = vegetation-stand with largest angular size), and of the entrances 1 to 3. For more than 80% of the nests, 40–90% of the circumference were surrounded by vegetation (Fig. 2). The mean angular size of the total vegetation-stand was 212° on Mellum and 205° on Memmert (Tab. 1). The nest entrances had a smaller angular size than the vegetation-stands (Tab. 1, Fig. 3). This shows the Herring Gull's preference for nest sites protected by surrounding vegetation.Vegetation-stand 1 was preferably orientated towards the west (= main wind direction), nest entrance 1 towards the east (Tab. 2, Fig. 4). The distribution of the total vegetation showed corresponding preferred directions (Fig. 6). A comparison of the years 1979 and 1980 revealed a shift of 50°–60°, from west to north, for the preferred direction of vegetation-stand 1 (Tab. 2, Fig. 4). A similar shift could be found for the mean wind direction (Fig. 5). For the two study plots on Mellum, a nest site selection favouring visual isolation from neighbours' nests could not be demonstrated (Tab. 3). The possible function of nest-surrounding vegetation as a shelter against wind and weather is discussed.

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Herrn Dr. Friedrich Goethe zur Vollendung des 70. Lebensjahres gewidmet