We have isolated a strain of Escherichia coli K-12 carrying a mutation, polA12, that results in the synthesis of a temperature-sensitive deoxyribonucleic acid (DNA) polymerase I. The double mutants polA12 recA56 and polA12 recB21, constructed at 30 C, are inviable at 42 C. About 90% of the cells of both double mutants die after 2 hr of incubation at 42 C. Both double mutants filament at 42 C and show a dependence on high cell density for growth at 30 C. In polU12 recB21 cells at 42 C, DNA and protein synthesis gradually stop in parallel. In poUA12 recA56 cells, DNA synthesis continues for at least 1 hr at 42 C, and there is extensive DNA degradation. The results suggest that the primary lesion in these double mutants is not in DNA replication per se.
1. A survey of acidified streams in the Loch Ard area of The Trossachs, Scotland, was carried out between 1986 and 1988, to determine the influence of physical and chemical factors on the distribution of benthic algae. Samples were taken on thirty-Bve occasions from fifteen sites on ten streams. Forty-nine operational taxa of filamentous algae were distinguished. Relative abundance of taxa in samples was scored on a seven-point scale.Estimates of standing crop were made by pigment extraction and ash-free dry weight (AFDW) determination from biomass developed on artificial substrata. 2. No evidence was found for seasonality in standing crop, nor for an increase in standing crop with a decrease in pH. Seasonality in relative abundance was evident for few taxa. Species richness and diversity were highly correlated with pH and correlated chemical variables. 3. Canonical correspondence analysis (CCA) carried out using the CANOCO program ordinates species and sites and relates them directly to environmental variables. The most important variable was found to be pH (or an acidification-related variable that is highly correlated with pH), with percentage forest cover and total oxidized nitrogen being of secondary importance. The effect of subdividing the data set, using data based on presence-absence as well as estimated relative abundance, was tested. A comparison of the effects on the distribution of site and species values on the pH vector showed that the results are robust, confirming the value of this semi-quantitative sampling method for use with a difficult algal group. It was possible to infer pH from algal community structure using CANOCO.4. The pH ranking of spedes derived from CCA revealed that larger cell diameter taxa do not predominate at low pH, implying that reduction of invertebrate grazing was not an important determinant of algal community compujsition.
SUMMARYWe have used the technique of in situ nucleic acid hybridization and autoradiography of thin frozen sections of human tissue to search for virus RNA sequences in human cervical tumours. Of cervical biopsies with abnormal cytology, 67 % bound herpes simplex virus type 2 (HSV2) 3H-labelled DNA probes and 39 % bound adenovirus type 2 (Ad2) 3H-labelled DNA probes, whereas control experiments with phage lambda 3H-labelled DNA probes, under the same conditions, bound to only 7 % of cases. In contrast, normal cervical biopsies bound the three probes in only 23 %, 17 % and 8 % of cases respectively.
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