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Biomedical subjects

J Kiefer

Publications and source records attributed to J Kiefer.

At least 145 records · Page 8Linked to original sources
PubMed1977

Synthesis of inducible enzymes in irradiated yeast cells. Ultraviolet effects on transcription and the influence of recovery processes.

The induced synthesis of arginase was measured in several yeast strains after ultraviolet light irradiation. There was an exponential dose-related reduction which was the same in all cell lines tested. This sensitivity is compatible with the size of the arginase structural gene if it is assumed that one pyrimidine dimer suffices for blocking transcription. The ultraviolet-induced synthesis inhibition is susceptible to photoreactivation and liquid holding recovery. The latter process is absent in a rad2 mutant and reduced in a rad9 mutant.

Arginase↗
PubMed1977

Split-dose and liquid-holding recovery after X-irradiation on diploid yeast Saccharomyces cerevisiae. I. Dependence on growth-phase.

Liquid-holding recovery (LHR) and the sparing effect of dose fractionation were investigated with 100 kVp-X-rays in diploid yeast from exponential and stationary phase. Exponential cells showed prompt and complete split-dose recovery. The ability to undergo LHR remained constant during 2-5 hours incubation after the first dose. Stationary-phase cells did not exhibit split-dose recovery (SDR) in a simple way: there was an increase in sensitivity after 3-5 hours followed by a gradual increases with time. It is suggested that stationary cells are starved of an essential co-factor which has to be synthesized de nova.

DNA Repair↗
PubMed1977

Split-dose and liquid-holding recovery after X-irradiation in diploid yeast Saccharomyces cerevisiae. II. Dependence of the recovery process on cellular protein metabolism.

In diploid yeast, split-dose recovery (SDR) after X-irradiation was affected, if incubation between split doses was performed in the presence of the protein-synthesis inhibitor, cycloheximide. In exponentially-growing cell-cultures, early SDR was undisturbed but complete recovery was not achieved. Concomitantly the cells show a decresing ability to perform subsequent liquid-holding recovery (LHR). In stationary-phase cell-cultures, SDR was completely suppressed. The cells show, however, recovery from potentially lethal damage in the presence of cycloheximide during incubation between the dose-fractions. The experimental results suggest that in diploid yeast SDR after X-irradiation is an enzymatic process dependent on a functioning protein metabolism.

Cell Survival↗
PubMed1976

Ultraviolet-induced inhibition of ribosomal RNA synthesis in yeast strains differing in radiation sensitivities.

The ultraviolet-induced inhibition of rRNA synthesis has been measured during the first hour after irradiation for stationary yeast cells differing in radiation sensitivity. rRNA was isolated and separated on an agarose-polyacrylamide gel. The wild type and a mutant which is possibly defective in recombinational repair show a sigmoidal inhibition curve, an excision-deficient mutant shows an exponential one. From these curves it is deduced that a pyrimidine dimer acts as a transcription terminating lesion as was shown for bacteria. During the first hour after irradiation the excision repair system decreases the number of transcription terminating lesions by 22% in the wild type and 25% in the mutant defective in recombinational repair. An approximation of the repair efficiency gives a value of 7500-10 000 transcription terminating lesions per cell being removed during the first hour after irradiation by excision. Ultraviolet-induced lesions of this kind can partially be removed by photoreactivation. The inhibition coefficients are the same for 26 S and 18 S rRNA in stationary cells, whereas exponentially growing cells show different inhibition coefficients for 26 S and 18 S rRNA leading to the suggestion that the processing of the ribosomal precursor RNA is different in stationary and exponentially growing cells.

Cell Division↗
PubMed1975

The effect of caffeine on survival of UV-irradiated diploid yeast strains of different sensitivities.

The action of caffeine post-treatment after UV exposure in three strains of diploid yeast has been studied. The addition of the drug to the plating medium reduced survival in all cases. Higher colony-forming abilities were found in all strains when the UV dose had been split into two fractions with an interval of about 6 h. Caffeine added to the incubation medium between dose fractions did not interfere with this process in the wild type but suppressed split-dose sparing completely in two sensitive strains.

Caffeine↗