Marine Ecosystem Science on an Intertwined Planet

Ecosystems - Marine ecosystem science has developed since the 1940s, when humans obtained the ability to spend substantial time underneath the surface of the ocean. Since then, and drawing on...     

Cytokinin Activity in Begonia and Bryophyllum

Cytokinin activity has been obtained in ethanol extracts of Begonia and Bryophyllum plants. Begonia x cheimantha Everett yielded 30 to 300 μg kinetin equivalents per kg of fresh leaves in the tobacco callus bioassay. Short day conditions appeared to increase the extractable cytokinin content in the tissue. Purification by fractionation on Dowex 50 H⁺ columns followed by organic solvent extraction, silver precipitation, and repeated paper chromatography yielded an apparently homogeneous product which accounted for most of the activity in the Begonia extracts. It was indistinguishable from zeatin in chromatograms developed with six solvent systems. Other cytokinin active fractions were also obtained from both Begonia and Bryophyllum. Crystalline picrate preparations of active products were insufficient for identification by mass spectrometry.     

Chromatin structure studied by linear dichroism at different salt concentrations

We have studied the linear dichroism (LD) of rat liver chromatin oriented by flow. Soluble chromatin, prepared by brief nuclease digestion, is found to exhibit a positive LD at low ionic strength (1 mM NaCl), with a constant LD/A over the absorption band centered at 260 nm (A, isotropic absorbance). Several previous dichroism studies on soluble chromatin have been performed on sonicated materials and have given negative LD, probably due to the presence of uncoiled DNA. The positive dichroism can be interpreted in terms of a supercoil of DNA in chromatin with a pitch angle larger than 55°, and is, for example, consistent with a model where the cylindrical nucleosome core particles are stacked face to face in the chromatin filament. In contrast to the nuclease-digested chromatin, sonicated chromatin was confirmed to exhibit negative LD. This difference can be attributed to a partial uncoiling of the linker regions between the nucleosomes due to the shearing. The structural transition of chromatin to a compact form can be observed as a reduction of the positive LD of the nuclease-digested chromatin to almost zero in 0.1 M NaCl or in 0.1 mM MgCl₂. This transition is due to a decreased electrostatic repulsion between negative phosphate groups on the DNA chain. In the case of Na⁺, this can be explained as a screening effect due to the bulk concentration of Na⁺. With Mg²⁺ a considerably stronger effect may indicate a more localized binding to the phosphates. At ionic strengths higher than 0.5M NaCl, the dissociation of the histones from DNA leads to uncoiling of chromatin. The change in LD during this process shows that histone H1 contributes only to a small degree to the coiling of the DNA chain, whereas histones H3 and H4 play the major role in the coiling.