Cleavage of DNA by mammalian DNA topoisomerase II

Using the P4 unknotting assay, DNA topoisomerase II has been purified from several mammalian cells. Similar to prokaryotic DNA gyrase, mammalian DNA topoisomerase II can cleave double-stranded DNA and be trapped as a covalent protein-DNA complex. This cleavage reaction requires protein denaturant treatment of the topoisomerase II-DNA complex and is reversible with respect to salt and temperature. The product after reversal of the cleavage reaction remains supertwisted, suggesting that the two ends of the putatively broken DNA are held tightly by the topoisomerase. Alternatively, the enzyme-DNA interaction is noncovalent, and the covalent linking of topoisomerase to DNA is induced by the protein denaturant. Detailed characterization of the cleavage products has revealed that topoisomerase II cuts DNA with a four-base stagger and is covalently linked to the protruding 5'-phosphoryl ends of each broken DNA strand. Calf thymus DNA topoisomerase II cuts SV40 DNA at multiple and specific sites. However, no sequence homology has been found among the cleavage sites as determined by direct nucleotide-sequencing studies.

Attachment and ingestion of mycoplasmas by mouse macrophages. II. Scanning electron microscopic observations

Mycoplasmas adhere closely to the central region of the surface of mouse peritoneal macrophages in vitro. They do not appear connected to each other or the macrophage membrane, and they induce no change in the surface of the cell. After addition of antimycoplasma antibody, mycoplasmas show interconnections and the cell shows an increase occurrence of ruffled membrane and folding over the mycoplasmas. Large and small lacunae appear in the membrane at sites other than those taking in organisms, and the cell develops a diffusely granular appearance. These changes are associated with an increase in pinocytosis of horseradish peroxidase that is 85% above controls. Five minutes after addition of antibody, the macrophage appears contracted and engorged and has persistent membrane changes consisting of pits, openings, and membrane folds. Trypsin causes slow ingestion of surface mycoplasmas without the obvious membrane folding over organisms but with evidence of a predominantly invaginating process of phagocytosis. The macrophage surface has numerous microprojections, but is does not have the granular appearance seen after addition of antibody. Trypsin and Mycoplasma pulmonis antigen do not enhance macrophage pinocytic rates. (Am J Pathol 87:347-358, 1977).

Down's syndrome with X0/XY mosaicism

A 5-month-old Chinese infant with combined Down's syndrome and X0/XY mosaicism is presented. The phenotypical features of mongolism are classical and unmodified. The somatic effects of X0/XY mosaicism is that of incomplete masculinization with short phallus, hypospadias, bifid scrotum, urogenital sinus, and bilateral extra-abdominal infantile testicles. These features together with the absence of internal female organs and signs of Turner's syndrome in this patient are different from the previous reported cases of mixed gonadal dysgenesis.

Structure of the red fluorescence band in chloroplasts

Using Weber's method of "matrix analysis" for the estimation of the number of fluorescent species contributing to the emission of a sample, it is shown that the fluorescence(1) band in spinach chloroplast fragments at room temperature originates in two species of chlorophyll a. Emission spectra obtained upon excitation with different wavelengths of light (preferentially absorbed in chlorophyll a or b) are presented. Upon cooling to - 196 degrees C, the fluorescence efficiency increases about twentyfold. Two additional bands, that now appear at 696 and 735 mmicro, suggest the participation of four molecular species. Emission spectra observed at different concentrations of chloroplast fragments with excitation in chlorophyll a and b and excitation spectra for different concentrations of chloroplast fragments and measurements at 685 and 760 mmicro are presented. Two of the four emission bands may belong to pigment system I and two to system II. The 685, 696, and 738 mmicro bands respond differently to temperature changes. In the -196 degrees C to -150 degrees C range, the intensity of the 685 mmicro band remains constant, and that of the 696 mmicro band decreases twice as fast as that of the 738 mmicro band.