In yeast reversion studies, assay of the total number of cells is made by plating irradiated cells on agar plates containing yeast extract, peptone and dextrose (YEPD) medium. The number of revertants are scored by plating cells on synthetic complete (SC) medium deficient in the particular nutrient for which the reversion is tested. In this procedure equivalence for cell survival between the YEPD and the SC media is always assumed. However it is shown in this paper that this assumption is valid only up to dose levels where cell killing is not significant. At high doses, survivals on the two media differ significantly from each other for both high and low LET radiations. This difference influences the slope of the reversion frequency curve at high doses. Since the reversion frequency is expressed with reference to the number of survivors after a given radiation dose, it is essential to see that the same chance of survival is offered to the reverted and unreverted cells. Even though reversion is reported to vary linearly with dose, it is found that this linearity is restricted only to dose levels where cell killing is not significant. At higher doses, the reversion frequency varies in a very complex manner with dose for both high and low LET radiations. The complexity depends further on the reference medium chosen.

Alpha Particles↗

PubMed1975

Post-irradiation modifications of radiation-induced heteroallelic reversion in diploid yeast: effect of nutrient broth.

The effect of post-irradiation growth in complete rich medium on the expression of the reversion to arginine-independence induced by gamma and alpha radiation in a heteroallelic diploid yeast strain (Saccharomyces cerevisiae BZ34) has been studied. During the post-irradiation treatment the reversion frequency increased, reached a peak at about 90 min and decreased thereafter reaching a constant value for treatment periods exceeding 6 h. As determined by the increase in number of budding cells, extensive DNA synthesis took place in cells incubated only in the nutrient medium and not in the omission medium. Hence the observed increase in the reversion frequency is explained on the basis that post-irradiation DNA synthesis is necessary for the expression of gene conversion. The decrease in the reversion frequency for continued treatment with yeast extract, peptone, dextrose (YEPD) is related to the fact that only one daughter of the post-irradiation first cell division is a revertant. The broth effect was not lost when the irradiated cells were first incubated for 90 min in arginine-less medium and then transferred to the broth. Similarly, the broth effect persisted even at doses high enough to induce considerable division delay. These results suggest that the radiation-induced pre-conversional lesions are not susceptible to repair by alternative pathways.

Alleles↗

PubMed1975