Critical importance of microsome concentration in mutagenesis assay with V79 Chinese hamster cells.

For optimum mutagensis in V79 Chinese hamster cells, the amount of liver postmitochondrial fraction in the assay was found to be of critical importance, depending on the chemicals being tested. Benzo[a]pyrene (BP) required lower (1-5%) concentrations of the liver 15 000 X g supernatant (S15) from methylcholanthrene pretreated rats for a maximum induction of cytotoxicity and mutagenicity, as determined by 8-azaguanine- and ouabain-resistance. A sharp peak of mutagenicity and cytotoxicity was induced by 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (7,8-diol BP) at a concentration of 1% of the S15 fraction. Little or no response was induced by these compounds with the S15 concentrations of more than 10%. Similarly, aflatoxin B1 induced a sharp peak of mutagenicity and cytotoxicity at a concentration of 2% of the liver S15 fraction from Aroclor-pretreated rats. Under the same condition, non-carcinogenic aflatoxin G2 did not induce cytotoxicity and mutagenicity. Analysis of BP metabolites by high-pressure liquid chromatography indicates that with the 30% S15 fraction, more than 80% of BP was metabolized during the first 15 min, while with the 2% S15 fraction, 7,8-diol BP increased continuously throughout the 120-min incubation period, suggesting a strong metabolic competition to rapidly remove BP and 7,8-diol BP with a high concentration of the S15. In contrast with these compounds, N-nitrosodimethylamine induced mutagenicity and cytotoxicity which increased linearly in proportion to the increasing amount of the S15 fraction from phenobarbitone- and Aroclor-pretreated rats. Various nitrosamines with different lipophilicity were examined at a high (30%) and low (2%) concentration of the S15 fraction from Aroclor-pretreated rats, in which ratios of mutation frequencies at 30% and 2% correlated inversely with lipophilicity of the compound. This result suggests that the lipid solubility of test compounds may be one factor which determines the concentration of post-mitochondrial supernatant for optimum mutagenesis.     

The Rb tumor suppressor is required for stress erythropoiesis

The retinoblastoma tumor suppressor gene plays important roles in cell cycle control, differentiation and survival during development and is functionally inactivated in most human cancers. Early studies using gene targeting in mice suggested a critical role for pRb in erythropoiesis, while more recent experiments have suggested that many of the abnormal embryonic phenotypes in the Rb null mouse result from a defective placenta. To address this controversy and determine whether Rb has cell intrinsic functions in erythropoiesis, we examined the effects of Rb loss on red cell production following acute deletion of pRb in vitro and under different stress conditions in vivo. Under stress conditions, pRb was required to regulate erythroblast expansion and promote red cell enucleation. Acute deletion of Rb in vitro induced erythroid cell cycle and differentiation defects similar to those observed in vivo. These results demonstrate a cell intrinsic role for pRb in stress erythropoiesis and hematopoietic homeostasis that has relevance for human diseases.     

Observations on the distinction between oligotrophic and eutrophic marine bacteria

The nutritional requirements of two marine bacteria designated as oligotrophic because they could grow on media containing 10 mg of C per liter supplied as peptone and two classified as eutrophic because they could grow only at higher concentrations of C supplied as peptone were examined. Each of the four organisms was found to have its own unique group of compounds which could serve either individually or in combination as sources of carbon and energy for growth. When the peptone in the medium was replaced by another appropriate source of carbon and energy, the difference in the capacity of the organisms examined to grow at 10 mg of C per liter disappeared, and all four organisms could be described as being oligotrophic. Some of the organisms required a low concentration of one specific carbon source but a higher concentration of another. One of the organisms was inhibited by high concentrations of one specific carbon source but not by another. The observations indicate that current methods of enumeration based on the capacity of cells to grow in the presence of high or low concentrations of complex mixtures of nutrients such as peptone do not distinguish between two broad classes of bacteria differing intrinsically in their ability to grow at high and low concentrations of nutrients. Whether two such broad classes exist seems extremely doubtful. Which organisms will multiply in a particular environment will depend on both the specific nutrients available and their concentrations.     

Effect of N,P, and K on root yield and nutrient levels in the leaves and roots of rutabagas grown in a greenhouse

Summary Experiments were conducted to determine the influence of N, P, and K and of the NP and NK interactions on root yields and tissue concentrations of N, P, and K of rutabagas grown on a sandy loam soil under greenhouse conditions. Root yields were increased by applications of N and K but not by added P. The yield response to N was dependent on K supply. The highest dry matter content in roots was associated with the lowest rates of N and K and the lowest root yields. The N content of tissue at the early vegetative stage increased with increasing rates of N and decreased with increasing rates of P and K. The N content of root tissue at harvest increased at the highest rates of N but was unaffected by rates of P and K. The P and K content of root tissue increased with increasing rates of P and K, respectively. The levels of nutrients in early vegetative tissue associated with optimum yields were about 2.6, 0.24, and 2.0% for N, P, and K, respectively. The corresponding values in the leaf tissue at harvest were about 1.2, 0.12, and 1.5% N, P, and K, respectively. Contribution No.222.     

THE POTENTIAL AND RESPIRATION OF FROG SKIN : I. THE EFFECT OF THE HOMOLOGOUS CARBAMATES. II. THE EFFECT OF CERTAIN LYSINS

Measurements of the O₂ consumption and of the potential of frog skin, made under comparable conditions, show that the homologous carbamates (ethyl, propyl, butyl, and amyl) reduce both the O₂ consumption and the potential, but not in a similar manner. In this respect, the effect of the carbamates is like the effect of reduction in O₂ tension. The simple lysins (saponin and the bile salts), on the other hand, abolish the potential without reducing the O₂ consumption at all. Irrespective of whether one considers the concentration of carbamate in the entire system or the amount of carbamate adsorbed by the frog skin, Traube''s rule relating the effect of a carbamate to its position in the homologous series does not seem to apply.     

Effect of Compost and Manure Soil Amendments on Nematodes and on Yields of Potato and Barley: A 7-Year Study

A 7-year study located in Prince Edward Island, Canada, examined the influence of compost and manure on crop yield and nematode populations. The compost used in this study consisted of cull waste potatoes, sawdust, and beef manure in a 3:3:1 ratio, respectively. No plant-parasitic nematodes were detected in samples collected from windrow compost piles at 5- and 30-cm depths prior to application on field plots. Low population densities of bacterial-feeding nematodes were recovered from compost windrows at the 5-cm depth. Field plots of potato (Solanum tuberosum cv. Kennebec) received compost applied at 16 metric tonnes per hectare, or beef manure applied at 12 metric tonnes per hectare. An adjacent trial with barley (Hordeum vulgare cv. Mic Mac) received only the compost treatment. In both trials the experimental design was a complete randomized block with four replicates. Data averaged over seven growing seasons indicated that population levels of root-lesion nematodes (primarily Pratylenchus penetrans) were higher in root-zone soil in potato plots treated with either compost or manure compared to the untreated control plots. The soil amendments did not affect root-knot nematode (Meloidogyne hapla) population densities in the potato plots, but clover-cyst nematodes (Heterodera trifolii) were more numerous in the root-zone soils of barley treated with compost compared to the untreated plots. Numbers of bacterial-feeding nematodes (primarily Diplogaster lheritieri) were greater in soil in potato plots treated with manure and in soil around barley roots than in untreated plots. Total yields of potato tubers averaged over seven growing seasons increased by 27% in the plots treated with either compost or manure. Grain yields of barley also were increased by 12% when compost was applied. These results indicated that organic amendments increased crop yields, but the impacts on different nematode species varied and usually increased soil population levels.