Enzymatic RNA synthesis on irradiated DNA

Radiobiology at the forefront: Hanns Langendorff and two of his disciples

Hanns Langendorff (1902-1974) was an eminent radiobiologist and a visionary, who not only helped found the field, but also made significant scientific contributions. He was a member of the first editorial board of IJRB and actually published a paper in its first issue about the radio-protector 5-hydroxytryptamine. Langendorff started working in the field of radiobiology in 1929 and became director of the 'Radiologisches Institut' of Freiburg University in 1936. His studies impressively show the development of radiobiology over decades in areas such as radiation-induced cell death at various stages of development, as well as radiosensitivity of sea urchin, yeast and mammals. Using mice, Langendorff made many early discoveries about spermatogenesis, hematopoiesis, prenatal development, chromosomal damage and metabolic pathways after exposures to X-rays and neutrons. He also investigated aspects of target theory and dosimetry and developed personal dosimeters using films. After the atomic bomb catastrophes in Japan, Langendorff and his collaborators soon began research in mice related to acute radiation sickness and stimulated the development of radioprotectors by studying their mechanisms of action associated with cell death, as well as cellular and metabolic changes involved. Langendorff also trained a cadre of young scientists who advanced the field and brought it to its golden age in the seventies and the eighties. Research activities of two of his disciples are reviewed: Ulrich Hagen and the author. Both made significant contributions: Hagen mainly studying DNA-damage and repair in vitro as well in cells and the author investigating metabolic processes, cellular and chromosomal damage, prenatal effects, genomic instability, individual radio-sensitivity and their connections to cancer therapy.

Biochemical aspects of radiation biology

In order to analyze the mechanisms of biological radiation effects, the events after radiation energy absorption in irradiated organisms have to be studied by physico-chemical and biochemical methods. The radiation effects in vitro on biomolecules, especially DNA, are described, as well as their alterations in irradiated cells. Whereas in vitro, in aqueous solution, predominantly OH radicals are effective and lead to damage in single moieties of the DNA, in vivo the direct absorption of radiation energy leads to 'locally multiply-damaged sites', which produce DNA double-strand breaks and locally denatured regions. DNA damage will be repaired in irradiated cells. Error free repair leads to the original nucleotide sequence in the genome by excision or by recombination. "Error prone repair"(mutagenic repair), leads to mutation. However, the biochemistry of these processes, regulated by a number of genes, is poorly understood. In addition, more complex reactions, such as gene amplification and transposition of mobile gene elements, are responsible for mutation or malignant transformation.

Inhibition of ribosomal RNA synthesis in yeast by ionizing radiations

The effects of ionizing radiations on transcriptional activity were studied in the diploid yeast Saccharomyces cerevisiae. As experimental indicator the synthesis of ribosomal RNA (r-RNA) was measured for 1 h after exposure of cells to gamma-rays, X-rays or alpha-particles. gamma- or X-ray induced transcription inhibition was always found to decrease exponentially with dose. D0 values of 2150 or 1950 Gy were determined in wild-type cells, corresponding to a mean energy of about 60 eV per r-RNA gene. The finding of differential sensitivities of the two high molecular-weight r-RNA species which are cotranscribed from r-DNA is compatible with the existence of a transcription terminating mechanism. Cells from a mutant strain (rad-9), radiation sensitive with respect to colony forming ability, show an approximately equal sensitivity for transcription inhibition compared to the wild-type (D0 (2095) = 2400 Gy). Inactivation of r-RNA synthesis in cells exposed to alpha-particles at room-temperature showed a decreased sensitivity with higher particle fluences ('resistant tail'). This phenomenon, however, was drastically reduced if the temperature during irradiation was lowered to 4 degrees C, and was completely abolished when dried cells were used. The results may be explained by the presence of recovery processes in vegetative cells during prolonged exposures to alpha-particles at room temperature. An inactivation cross-section for alpha-particle induced transcription inhibition of about 0.02 micron 2 can be derived from the experimental data. When comparing this value with the estimated target volume, it appears that no single genes but the whole cluster of about 120 r-RNA genes on each of the chromosomes XII in the diploid cell may be inactivated by a single alpha-particle.

Oxygen-effect as an inhibition of repair: radiation studies on excision repair deficient mei-9L1-embryos of Drosophila

The excision repair deficient mei-9L1-embryos of Drosophila melanogaster are up to four times more radiosensitive than normal +/+ embryos. The lack of oxygen-effect in the repair deficient 4-h-embryos and the reduced O2-effect in the 13/4-h embryos suggest an interpretation of the oxygen effect as a modification of the ability to repair. The conversion of the early death (heavy damage) to late death (slight damage) by irradiation of normal embryos in N2 supports this interpretation. This theory can also explain the dependence of O2-effect on LET. The spontaneous lethality and the increase in radiosensitivity depend in heterozygous mei-9L1-embryos strictly upon the genotype of the mother, thus representing a maternal effect.

An apparent correlation between the inhibition of induced ornithine decarboxylase activity by gamma radiation and the capacity for DNA repair synthesis in normal and ataxia telangiectasia human fibroblasts: no correlation with cell survival

Exposure of normal human fibroblasts (F107) in stationary phase to gamma radiation inhibited the appearance of induced ornithine decarboxylase (ODC) activity. Skin fibroblasts derived from two ataxia telangiectasia (AT) patients (F184 and F182) displayed a similar response. The level of DNA repair synthesis was also similar in the three cell strains. Fibroblasts from another apparently normal donor (F196) were very sensitive to inhibition of induced ODC activity by gamma radiation and were also deficient in radiation-induced DNA repair synthesis. However, the two strains derived from normal donors displayed the same degree of cellular sensitivity towards X-rays, whereas the two AT strains showed the typical hypersensitivity to the cytotoxic effect of X-irradiation. The results suggest a possible correlation between the inhibition of induced ODC activity by gamma radiation and the extent of DNA repair synthesis at high radiation doses, but there is no correlation between these two parameters and cellular survival at low radiation doses.

The effects of various kinds of sparsely-ionizing radiation on total cell RNA and preribosomal nuclear RNA of Novikoff hepatoma ascites cells after in vivo labelling

The effects of 300 kV X-rays, 60Co gamma-rays, 43 MV X-rays and 43 MeV electrons on total cell RNA and 45 S pre-r RNA were investigated in Novikoff Hepatoma ascites cells. Six hours after local irradiation of the animals with 600 rad, the tumour cells were labelled in vivo for 30 and 60 min. The specific activities of high-molecular-weight RNA were influenced differently. This may be due to divergent effects of the applied radiation qualities at the level of transcription and transformation (processing). In addition, these radiation effects are mediated by the adrenal glands in a rather complex manner. Therefore, the results presented in this study support the suggestion that an in vivo labelling system is unsuitable for the evaluation of quantitative radiation effects on RNA.

Template activity of calf thymus DNA modified by a dihydrodiol epoxide derivative of benzo[a]pyrene

The purpose of the present study was to determine the effects of covalent binding to DNA of a reactive derivative of benzo[a]pyrene on template activity during in vitro transcription with RNA polymerase. Calf thymus deoxyribonucleic acid, modified by reaction with (+/-)-7beta,8alpha-dihydroxy-9alpha, 10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, was transcribed with Escherichia coli DNA-dependent RNA polymerase. With increasing levels of modification, there was a progressive inhibition of transcription. The inhibition was much greater under conditions where continuous reinitiation of transcription occurred than under conditions where only one RNA chain was synthesized per initiation site. This suggested that the modified sites block the movement of polymerase along the template and prevent recycling of the enzyme. Consistent with this interpretation were analyses of RNA transcripts on sucrose density gradients which showed a progressive decrease in average RNA chain length as the extent of template modification increased. In contrast to the inhibitory effect on chain elongation, evidence was obtained that the modified DNA had an increase in the number of initiation sites for transcription. These results are consistent with separate physical studies indicating that modification of DNA by this benzo[a]pyrene derivative can induce small localized regions of denaturation.

RNA synthesis in Yoshida ascites tumor cells after ultraviolet and gamma irradiation

The residual RNA synthesis in Yoshida ascites tumor cells following exposure to ultraviolet radiation was insensitive to rifampicin AF/013, an inhibitor of initiation of RNA synthesis in eukaryotic cells. On the other hand, residual RNA synthesis in the ascites tumor cells after gamma-irradiation was inhibited by rifampicin AF/013. In vitro synthesis of RNA on ultraviolet-irradiated ascites chromatin likewise was resistant to rifampicin AF/013. Ultraviolet irradiation was found to decrease the number of RNA chain initiation sites on ascites chromatin considerably whereas the number of initiation sites on ascites chromatin increased considerably after gamma irradiation. These results indicate that ultraviolet irradiation may preferentially affect the DNA segments participating in initiation of RNA synthesis.

Inhibition of transcription by isonicotinic acid hydrazide

Isonicotinic acid hydrazide (INH) reacts with the CMP moiety of a polynucleotide at slightly acid and alkaline pH. The reaction product, when used as a template in the cell-free transcription step, greatly diminishes the incorporation of GMP. In this system we were not able to show that INH -- in contrast with hydrazin -- is potentially capable of producing point mutations, since noncomplementary incorporation could not be observed with a poly(C)/INH adduct as template. Hydrazine could not be detected when INH was incubated with liver-cell fractions.

Recovery of the priming activity of DNA in x-irradiated Escherichia coli

The template activity of DNA in RNA synthesis in vitro has been studied in Escherichia coli B/r and Bs-1 after exposure to X-rays and postirradiation incubation in growth medium for 60 min at 37 degrees C. The incubation of E. coli B/r after irradiation with 9.3 krad results in the increase of the priming activity of DNA practically to that of unirradiated cells, while after exposure to 18.6 krad the incubation leads to a partial increase in its priming activity. As for E. coli Bs-1, the incubation of the bacteria irradiated with 9.3 krad causes a slight recovery in the priming activity of DNA.

Use of bacteriophage phi chi 174 replicative from progeny DNA as templates for transcription

The synthesis of phiX174-specific RNA has been studied in infected cells in which the thymine of the viral (+) strand of the parental RF(*), of the complementary (-) strand of the parental RF, or of both strands of the progeny RF molecules has been replaced with 5-bromouracil (5 BU). By irradiation of such cells with UV light at a wavelength of 313 nm it was possible to affect, specifically, the 5 BU-labeled strands. When the progeny RF molecules contain thymine, irradiation has no effect upon the synthesis of viral-specific RNA, regardless of 5 BU substitution in either strand of parental RF. If, however, progeny RF is labeled with 5 BU, irradiation produces a major decrease of viral RNA synthesis. It is concluded that many progeny RF molecules can serve as templates for transcription at late times of infection. Irradiation, prior to RF replication, of cells in which, particularly, the complementary strand of RF contains 5 BU, appears to decrease the ability of the parental RF to replicate.