Thermal denaturation of calf thymus DNA: existence of a GC-richer fraction

In 2.5 x 10(-4)M EDTA buffer, the derivative melting curve of calf thymus DNA shows a major band at 47 degrees with a shoulder at about 54 degrees . The fraction of melting area of this shoulder is about 13%. For reconstituted polylysine-calf thymus DNA complexes, in addition to the melting of free DNA regions at about 50 degrees (T(m)) there is another melting at about 106 degrees (T(m)) of polylysine-bound regions. The melting band of the complex at T(m) is not symmetrical. As more polylysine is bound to DNA the melting amplitude is diminished greatly on the major band at 47 degrees but only slightly on the shoulder at 54 degrees . The insensitivity of this shoulder appears to result from the existence of a 13% fraction of calf thymus DNA containing 55% GC. It is not favorably bound by polylysine. It remains in the supernatant after centrifugation and melts at about 54-56 degrees . This conclusion is further supported by two facts: the reconstitution method provides a condition for selective binding of polylysine to AT-rich DNA, and it yields a fully symmetric melting band at T(m) for complexes of polylysine with homogeneous bacterial DNA such as the one from M. luteus.     

A comparison between methods of calculating skeletal age (Greulich-Pyle)

The skeletal age of each individual bone of the hand-wrist was assessed in serial radiographs of 169 Melbourne children. Seven different methods were used to obtain area skeletal ages from these bone skeletal ages. Methods employing arithmetic means of all the bone skeletal ages, excluding only extreme values, excluding those derived from the carpals or from other selected bones yielded similar means during the age range studied in each sex. There were large, but not significant, differences between the means derived from averages of the skeletal ages of the most and least mature bones and those derived from all the bone skeletal ages. A system of weighting and exclusion of bone skeletal ages led to the recording of area skeletal ages that differed from the others and fluctuated markedly for individuals until ossification had occurred in every carpal bone. The findings suggest that the exclusion of the skeletal ages of the carpal bones or the use of selected skeletal ages could lead to quicker assessments without real changes in means and variability.     

Comparison of the effects of ultraviolet light and ethylmethanesulphonate upon the frequency of mitotic recombination in yeast

Summary Host-cell reactivation of gamma-irradiated phage T1 in strains of E. coli K-12 has been compared with HCR of UV-irradiated phage in these same strains and with the radiation sensitivities of these strains (Fig. 1–4). The pattern of the HCR of gammairradiated phage in these strains is like that of the HCR of UV-irradiated phage. HCR in strains whose genotype is uvr ⁺rec^- is like that of the wild type; whereas, HCR is minimal in strains which are uvr ^-. It is suggested that some type of gamma-ray-induced base damage in phage DNA is repaired in uvr ⁺ strains.This work was supported by the United States Atomic Energy Commission Contract No. AT(11-1)-1686. — This is report No. COO-1686-6.Supported in part by the United States Public Health Service Training Grant No. 5T1 RH-80-02(67).     

Influence of Calcium and Magnesium on Manganese Absorption

Mutual effects between Mn, Ca, and Mg were studied during steady-state absorption experiments with excised barley roots. Calcium appeared to enhance the rate of Mn absorption; whereas, Mg had a highly depressive effect. The combination of both Ca and Mg was even more inhibitory to Mn absorption than Mg alone. Manganese had no effect on the usual negligible Ca absorption by this tissue, but effectively inhibited the absorption of Mg. Although divalent cation absorption from the Ca-Mg-Mn system was essentially nil, K absorption was greatly stimulated in the presence of these cations.These mutual effects and others reported in the literature are explained by the hypothesis that selectivity in ion absorption results from cation-induced conformational changes in the structure of the carrier molecule.     

Use of Erythrocytes Sensitized with Purified Pneumococcal Polysaccharides for the Assay of Antibody and Antibody-producing Cells

A method was described for the sensitization of erythrocytes with purified type-specific pneumococcal polysaccharide antigens using chromium chloride as a coupling agent. Erythrocytes so sensitized can be used in routine passive hemagglutination and hemolysis tests as well as in the technique of localized hemolysis-in-gel for the detection of specific antibody and specific antibody-producing cells, respectively.     

Investigation of Reported Aflatoxin Production by Fungi Outside the Aspergillus flavus Group

A screening study of 121 fungus isolates, representing 29 species, for aflatoxin synthesis demonstrated this property only in Aspergillus flavus and A. parasiticus. Eight of the organisms found negative were isolates reported by other investigators to produce aflatoxin. Since similar negative reports have come from several other workers, it is concluded that only the A. flavus group of Aspergillus can presently be certified as sources of these toxins. Reasons for possible false-positive findings are discussed along with precautionary measures and differential analytical procedures useful in aflatoxin screening studies.