Effects of ionizing radiation on synchronous cultures of Escherichia coli B-r

Sensitivity to ultraviolet radiation as a function of DNA content in Escherichia coli B/r

Populations of Escherichia coli B/r A were grown to log phase at various growth rates determined by the richness of the medium. The genome content, G, was calculated from log phase doubling times by means of the Cooper-Helmstetter formula. Cell volumes were measured and found to vary linearly with this genome content. Cells with various DNA contents were prepared for ultraviolet irradiation and plated for dark repair under similar conditions. The resulting logarithmic survival curves were all similar in shape: convex up, with straight line portions having approximately the same slope (D0 = 11.4 +/- 0.2 J/m2). The shoulders however increase in width with calculated DNA content giving an extrapolation number which varies roughly as exp(G) or exp (0.6 Gmax).

Induction of prophage lambda during the division cycle of Escherichia coli

When synchronous populations of Escherichia coli B/r (lambda) were exposed to low doses of ultraviolet light, the yield of infective centres varied with cell age. The yield was highest if the lysogenic bacteria were irradiated at a time which coincides approximately with the termination of rounds of DNA replication and it was lowest when dividing cells were irradiated. No such variation was detected following either irradiation of excision-defective lysogenic cells or thermal induction of lambdacI857 prophage in irradiated bacteria. It is suggested that the variation reflects a relationship between prophage induction and inhibition of cell division. This hypothesis is supported by data showing that irradiation promoted induction and curtailed division in E. coli K12 dnaA mutants which were dividing in the absence of DNA replication.

Relation between survival and deoxyribonucleic acid replication in ultraviolet-irradiated resistant and sensitive strains of Escherichia coli B-r

When arabinose-grown Escherichia coli B/r is ultraviolet (UV) irradiated in the logarithmic phase of growth, the dose inactivation curve for both colony formation and deoxyribonucleic acid (DNA) synthesis (based on the relative rates of synthesis) is exponential in nature. When protein synthesis is inhibited before UV-irradiation, both inactivation curves have a large shoulder. Pre-irradiation inhibition of protein synthesis increases considerably the colony-forming ability of a UV-irradiated Hcr(-) and Rec(-) strain of E. coli B/r. However, with the repair-deficient strains, both the shoulder and slope of the survival curve are affected. We investigated the effect of UV irradiation on DNA synthesis in Hcr(-) bacteria and found that pre-irradiation inhibition of protein synthesis increases UV resistance of DNA replication in this strain also. The results suggest that inhibition of protein synthesis before irradiation increases UV resistance in E. coli B/r by a mechanism which is independent of both the excision and recombination repair systems.

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.

Regulation of deoxyribonucleic acid replication and cell division in Escherichia coli B-r

Synchronous cultures of Escherichia coli strain B/r were used to investigate the relationship between deoxyribonucleic acid (DNA) replication and cell division. We have determined that terminal steps in division can proceed in the absence of DNA synthesis. Inhibition of DNA replication with nalidixic acid prior to the start of a new round of replication does not stop cell division, which indicates that the start of the round is not essential in triggering cell division. Inhibition of DNA replication at any time prior to the termination of a round of replication completely blocks cell division, which suggests that there may be a link between the end of the replication cycle and the commitment of the cell to divide. Studies that use a temperature-sensitive mutant which is unable to synthesize DNA at the nonpermissive temperature are in complete agreement with those that use nalidixic acid to inhibit DNA synthesis. This adds support to the idea that the treatments employed limit their action to DNA synthesis. Investigation of minicell production indicates that the production of minicells is blocked when DNA synthesis is inhibited with nalidixic acid. Although nuclear segregation is not required for cell division, DNA synthesis is still required to trigger division. The evidence presented suggests strongly that (i) DNA synthesis is essential for cell division, (ii) the end of a round of replication triggers cell division, and (iii) there is considerable time lapse (one-half generation) between the completion of a round of DNA replication and physical separation of the cells.