Effects of broad-band vacuum-uv synchrotron radiation on wet yeast cells

By placing a layer of water vapor of near 100% relative humidity (20 to 30 Torr depending on room temperature) between cell samples (Saccharomyces cerevisiae) and a MgF2 window at the end of the vacuum path of the photon beam, we were able to irradiate cells under wet conditions with vacuum-ultraviolet light (115 to 190 nm) using synchrotron radiation from an electron storage ring (0.3 GeV). A deuterium discharge lamp with a MgF2 window was also utilized as a source of vacuum uv in a similar wavelength range. Survival decreased in a sigmoidal way. The induction of gene conversion (at trp5 locus) occurred by irradiation with synchrotron radiation to a much lesser extent on a survival basis compared with that by far-uv (254 nm) irradiation. The induction was, however, attributed to the far-uv component of synchrotron radiation based upon the observation of photoreactivation. A remarkable feature with vacuum-uv effects was that holding in liquid after irradiation brought about a drastic decrease of survival. Furthermore, the sensitivity to treatment after irradiation with toluidine blue (membrane-attacking agent) and visible light was found to be significantly higher in cells exposed to synchrotron radiation than to far uv. These results indicate that damage on or near the cell membrane is more important than that in the nucleus for cell inactivation by vacuum-uv radiation