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Biomedical subjects

E C Pollard

Publications and source records attributed to E C Pollard.

At least 37 records · Page 2Linked to original sources
PubMed1973

All-or-nothing character of DNA degradation in bacteria after ionizing radiation.

The response of single cells of Escherichia coli B(s-11) and 15JG151 to radiation-induced DNA degradation has been observed by autoradiography. For both cells it is concluded that the event which causes DNA degradation is of an all-or-nothing character. The unit which suffers degradation is not the whole cell, but each cell has between two and four such units. The results suggest that there is some resynthesis of degraded DNA. Evidence that this occurs is shown by examining the degradation of mass cultures of 15TAU/t3 and t7 below and above the permissive temperature for DNA synthesis. The results on the all-or-nothing character are in agreement with previous studies made by completely different techniques.

Autoradiography↗
PubMed1973

Degradation of cellular deoxyribonucleic acid after gamma irradiation of spheroplasted Escherichia coli CR thy- cells.

Gamma-ray-irradiated Escherichia coli CR thy(-) cells and spheroplasts, prelabeled with (14)C-thymine, were assayed for acid-insoluble activity as a function of incubation time after irradiation. Under similar irradiation and incubation conditions, degradation profiles of cells and spheroplasts were virtually identical. Similar results were found for cells and protoplasts irradiated in the presence of rifampin (20 mug/ml). These results suggest that postirradiation deoxyribonucleic acid degradation enzymes are probably not loosely localized in the periplasm, unlike endonuclease I.

Carbon Radioisotopes↗
PubMed1972

Action of ionizing radiation on sensitive strains of Escherichia coli B.

Strain B(8-11) has been found to be very sensitive to postirradiation DNA degradation. Up to 98% of the DNA is degraded at optimum doses. The amount of residual DNA correlates with the retention of colony-forming ability (CFA). Studies of rates of degradation as a function of dose agree with the concept that a degrading lesion causes a definite rate of degradation and that increased numbers of lesions produce proportionally faster rates. By observing the burst size of T7 phage which uses host DNA it has been established that DNA degradation occurs in an all-or-nothing fashion in a unit which is present two or three times per cell. Degradation is enzymatic and the enzyme system is already present in the cell as evidenced by the rapid onset of degradation. DNA synthesis continues in cells that have lost some chromosomes by degradation. Single-cell division patterns show that recovery from "sublethal" damage can occur even in this sensitive cell. Recovery in preirradiation oxygenated cells differs from that in nitrogenated cells.

Carbon Isotopes↗
PubMed1972

Hydrostatic pressure effects on protein synthesis.

The effects of high hydrostatic pressure on several phases of cell-free protein synthesis have been examined. The initial rate of polyuridylic acid (poly U)-directed synthesis of polyphenylalanine showed an apparent increase at 100 atm, above which the synthetic rate was reduced sharply with increased pressure up to 640 atm where 95% inhibition was observed. The magnitude of the inhibition of polyphenylalanine synthesis with increased pressure depended strongly on the magnesium salt concentration in the reaction system. Misreading of the poly U message, as measured by insertion of leucine in place of phenylalanine, dropped rapidly with increased pressure from 1 to 350 atm, above which the amount of misreading increased. Enzymatic activation of transfer RNAs (tRNAs) was reduced by increased pressure in the range 100-640 atm, where the rate of tRNA aminoacylation was 80% inhibited. Both nonenzymatic attachment of phenylalanyl-tRNA (phe-tRNA) to the poly U-ribosome complex and stability of the phe-tRNA-poly U-ribosome complex were decreased at high pressures (100-900 atm). The results of the action of pressure on the various phases of cell-free protein synthesis suggest that the major pressure-sensitive element in the protein synthetic machinery is the ribosome.

Amino Acyl-tRNA Synthetases↗
PubMed1972

Effects of ionizing radiation on the capacity of Escherichia coli to support bacteriophage T4 growth.

The loss in capacity of irradiated bacteria to support the growth of T4 phage has been studied for two strains, Escherichia coli B and E. coli B(s-1). Following a dose of 25 krads, the capacity is quite rapidly lost during postirradiation incubation so that after two hr of such incubation at 37 C only 12% remains in strain B and 3% in strain B(s-1). Evidence that capacity is lost in an all-or-none fashion was provided by two types of experiments: (i) a single-burst analysis of those cells which survived to give a burst, and (ii) an analysis of the regulation of an early phage enzyme. Several processes were examined to try to determine the cause of the loss in capacity. These were the ability of the irradiated and postirradiation incubated cells to respire, to allow phage adsorption and injection of phage deoxyribonucleic acid, to support the transcription and translation of an early gene, and to support replication of phage deoxyribonucleic acid. Of these processes, transcription and translation appeared to be the most closely associated with the capacity loss, although respiration was reduced to 50% after 2 hr.

Adsorption↗
PubMed1971

The thermal inactivation of T4 and lambda bacteriophage.

Thermal inactivation of T(4) and lambda bacteriophage shows two components of differing sensitivity are present. These cannot be interpreted as owing to nucleic acid and protein. One protein function-the inhibition of radiation-induced DNA degradation-is lost with quite different thermal kinetics. lambda heated in the presence of DNase is more rapidly inactivated; lambda is also protected by slow cooling after heat. These results suggest that the packing of the DNA in the head occurs so as to permit different degrees of thermal expansion in the outer coils. These can rupture the coat and this is one form of inactivation. Killed vaccines could be more safely made by heating in the presence of a nuclease followed by rapid cooling.

Coliphages↗
PubMed1969

Division patterns from single Escherichia coli cells.

A method has been worked out for studying the division of single bacterial cells (Escherichia coli B/r) in a uniform environment. Under optimal conditions the daughters of one single cell are found to divide at different times, a fact which indicates that they are not identical. The spread in generation times can be estimated quantitatively. When cells are irradiated with gamma rays in nutrient broth there is an increase in the spread in generation times, and the number of three-cell progeny (which require considerable difference between the daughters) rises. The results are consistent with the idea that damage to a segment of DNA has taken place and that there are three growing points on the DNA at any one time. In nutrient broth there is some evidence for repair of damage. For cells irradiated in minimal medium the pattern is different. The increase in generation times is not so marked, and only slight increase in three-cell progeny is seen. The results suggest that there is the same class of damage to the DNA, with only one growing point present and less repair. Using the criterion that no increase in generation time at all is permitted, we can estimate the total escape probability after radiation. The resulting calculation of critical target size is much closer to the whole bacterial chromosome than is found from normal plating experiments. The behavior of cells that have been exposed to irradiated medium is quite different, involving very long lags and cell death.

Cell Division↗
PubMed1969

The study of ionizing radiation effects on Escherichia coli by density gradient sedimentation.

Density gradient sedimentation of bacterial cells in cesium chloride has been used to separate cells which have been irradiated with (60)Co gamma rays and have lost an appreciable amount of their DNA by subsequent degradation. Irradiated cells are found to band mainly at two characteristic densities, one corresponding to normal unirradiated cells and the other at a considerably lower density. The region corresponding to normal density cells is the only one that contains cells which will form colonies. Cells capable of synthesizing DNA following irradiation are found mainly at the region of normal density cells with some spreading into the lower density region. Cells in the lower density region contain less DNA than normal density cells. From an analysis of the relative numbers of cells in the two regions, it is suggested that the process of DNA degradation either takes place to a considerable extent in the genome or not at all. Analysis of the data in terms of numbers of cells having intact DNA and those having degraded DNA indicates a strong correlation between DNA degradation and cell death in this strain, JG151, and suggests that DNA degradation is a major but not the only cause of cell death.

Bacterial Proteins↗
PubMed1969

A study of the effects of hydrostatic pressure on macromolecular synthesis in Escherichia coli.

In cultures of Escherichia coli 15 (thymine(-), leucine(-)) which were incubated at high hydrostatic pressures, cell division occurred only at pressures below 430 atm but in a somewhat synchronous fashion at around 250 atm. The rate of leucine-(14)C incorporation into a macromolecular fraction of the cells diminished to a zero value at about 580 atm and that of uracil-(14)C incorporation to a zero value at about 770 atm. The rate of thymine-(14)C incorporation at pressures around 330 atm was that to be expected with a culture in which DNA synthesis is somewhat synchronous. At pressures above 500 atm, thymine-(14)C was incorporated only over the initial part of the pressure incubation and further incorporation under pressure was not observed no matter how long the duration of the incubation. We present evidence along several lines that the thymine incorporation kinetics reflect an effect of pressure on a locus at the origin (or termination) of a replication of the bacterial chromosome. The recovery of cell division and of the incorporation rates upon release of pressure were found to depend on the magnitude of the pressure and the duration of the pressure incubation.

Bacterial Proteins↗