Variation of the Biological Effectiveness of X-Rays and α-Particles on Haploid Saccharomyces cerevisiae

Abstract

Log phase S. cerevisiae (haploid) grown in yeast extract, dextrose (YED) liquid medium and subsequently starved in a buffered (pH 4.5) dextrose medium deficient of any fixed N source were the subject of this study. Using 50 kv X-rays and Po210 [alpha]-particles degraded in energy to 3.4 mev the radiosensitivities and relative biological effectivenesses (RBE) of interdivisional cells were determined as a function of starvation time. For this purpose, survival curves determined from visible colony formation on YED agar plates were employed. It was found that: (1) the initial portion of the survival curves for a -particles and X-rays was always exponential; (2) [alpha] -particle as well as X-ray irradiation revealed a radioresistant moiety correlatable with the percentage of cells with small buds; (3) the shape of the survival curves are "multiple-hit" for the radioresistant moiety and the RBE can be > 1 when for the interdivisional cells in the same population the RBE < 1; (4) during a starvation period of 9 days, the RBE determined for interdivisional cells varied by a factor of 3 (from 0.66 to 2.0) resulting mainly from a 2.5 fold variation in the X-ray sensitivity; and (5) most of the variation described in (4) occurred within 3 days. It is concluded from these observations that: (1) if the sensitive sites are the same for [alpha] -particles and X-rays, then the RBE variation reflects variations in the sensitivity of the sites and not only changes in the atomic composition of the sites; and (2) the variation in RBE from less than to greater than one makes it appear doubtful that either the target or diffusion model theories can adequately explain these results.