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.     

Control of mixed-substrate utilization in continuous cultures of Escherichia coli

The chemostat culture technique was used to study the control mechanisms which operate during utilization of mixtures of glucose and lactose and glucose and l-aspartic acid by populations of Escherichia coli B6. Constitutive mutants were rapidly selected during continuous culture on a mixture of glucose and lactose, and the beta-galactosidase level of the culture increased greatly. After mutant selection, the specific beta-galactosidase level of the culture was a decreasing function of growth rate. In cultures of both the inducible wild type and the constitutive mutant, glucose and lactose were simultaneously utilized at moderate growth rates, whereas only glucose was used in the inducible cultures at high growth rates. Catabolite repression was shown to be the primary mechanism of control of beta-galactosidase level and lactose utilization in continuous culture on mixed substrates. In batch culture, as in the chemostat, catabolite repression acting by itself on the lac enzymes was insufficient to prevent lactose utilization or cause diauxie. Interference with induction of the lac operon, as well as catabolite repression, was necessary to produce diauxic growth. Continuous cultures fed mixtures of glucose and l-aspartic acid utilized both substrates at moderate growth rates, even though the catabolic enzyme aspartase was linearly repressed with increasing growth rate. Although the repression of aspartase paralleled the catabolite repression of beta-galactosidase, l-aspartic acid could be utilized even at very low levels of the catabolic enzyme because of direct anabolic incorporation into protein.     

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.