Specific Binding of Rubidium in Chlorella

Specific binding sites for potassium, which may be components of the carriers for active transport for K in Chlorella, were characterized by their capacity to bind rubidium. A dense suspension was allowed to take up Rb⁸⁶ from a low concentration of Rb⁸⁶ and a high concentration of ions which saturate non-specific sites. The amount bound was derived from the increase in the external concentration of Rb⁸⁶ following addition of excess potassium. The sites were heterogeneous. The average affinity of Rb and various other ions for the sites was determined by plotting the degree of displacement of Rb⁸⁶ against log molar concentration of the individual ions. Interpolation gave the concentration for 50 per cent displacement of Rb, which is inversely related to affinity. The order of affinity was not changed when the cells were frozen, or boiled either in water or in 70 per cent ethanol. The affinity is maximal for ions with a crystalline radius of 1.3 to 1.5 A and a high polarizability, and is not related to the hydrated radius or valency. It is suggested that binding groups in a site are rigidly arranged, the irregular space between them being 2.6 to 3.0 A across, so that affinity is high for ions of this diameter and high polarizability.     

EFFECTS OF PUROMYCIN ON THE NUCLEOPROTEINS OF THE HELA CELL

The effects of several concentrations of puromycin on the nucleoproteins of HeLa cells grown in monolayers were studied by cytochemical and biochemical techniques. The earliest change at all concentrations of puromycin was a decrease in a granular form of ribonucleoprotein (RNP) that is demonstrable in the normal HeLa cell by the toluidine blue-molybdate (TBM) stain. The other types of RNP revealed by the TBM method were unaltered although the cell volume decreased markedly. Treatment with high concentrations of the antimetabolite resulted in pre-prophase inhibition of mitotic division and led to production of inclusions containing RNP in the cytoplasm; lower concentrations resulted in metaphase arrest. Biochemical analyses confirmed the cytochemical observations and indicated that synthesis of RNA and protein was inhibited to the same extent.     

Circular dichroism measurements of benzyl L-aspartate–nitrobenzyl L-aspartate copolymers and their use in detecting and characterizing preferred polymer conformations

We have examined the nature of the circular dichroism band at 330 mμ for a series of copolymers of β-p-nitrobenzyl L -aspartate with β-benzyl L -aspartate. The circular dichroism band arises from an electronic transition in the nitroaromatic group. In order to interpret the effect quantitatively, we employed a simplified statistical treatment and curve fitting for six copolymers. Both approaches gave consistent results, which indicates that the dichroism comes from pairwise interactions between two nitrobenzyl groups. We constructed a molecular model that meets the constraints and requirements of the analyses developed in this paper. In this model, it is proposed that the main chain forms a right-handed α-helix and that nitrobenzyl groups separated by four residues interact with each other.     

THE ACROSOME REACTION IN MYTILUS EDULIS : I. Fine Structure of the Intact Acrosome

The intact acrosome of the Mytilus edulis spermatozoon consists of a conical vesicle, the basal side of which is deeply invaginated so that the whole vesicle forms a sheath around a very slender axial rod, about 2.7 µ long, inserted in a tube passing through the nucleus. The annular base of the acrosomal vesical is filled with a homogeneous substance; the outer wall of the vesicle is lined with a somewhat irregular layer of a particulate substance interspersed with very fine tubular elements, and its lumen is nearly filled by a strand of material which extends from the inner tip of the invagination to the apex of the acrosome. The lumen of the invagination appears empty except for the rod and a delicate sleeve-like structure which surrounds it. The plasma membrane of the sperm cell lies in immediate contact with the acrosomal membrane over its whole outer surface. In its general organization, this molluscan acrosome shows a rather close homology with that of the annelid Hydroides.     

Production of the Milk Agent in Cultures of Mouse Mammary Carcinoma

Thin sections of tissue cultures grown from tumors of the RIII high-breast-cancer strain mice were studied in the electron microscope. These tissues contain an abundance of particles whose morphology is consistent with biophysical measurement of the milk agent. These particles, found only extracellularly in our cultures, are formed at the cell membrane. The process of formation, as reconstructed from sections, appears to include a thickening and protrusion of the cell membrane which then evolves gradually into a dense sphere and separates from the cell in much the same manner as does influenza virus. The contents of the newly formed body are later rearranged to form a nucleoid within a membranous sac.