Starvation of protozoans increases their sensitivity to ultraviolet radiations and affects the photoreversibility of ultraviolet injury by visible light (Giese and Reed, 1940; Giese et al., 1954; Brandt et al., 1955). The present study on the variation with nutritional state of the sensitivity of yeast to ultraviolet and its photoreversal was initiated with the aim of relating the specific nutrients or enzyme systems involved in the changed ultraviolet sensitivity resulting from starvation. In addition the effects on ultraviolet injury and its photoreversal of some factors which affect the physiological state of the yeast, especially oxygen tension and selective enzyme poisons, were investigated; findings which emerged are presented below.

The Na + and K + content of non-metabolizing yeast cells was determined before and after monochromatic ultraviolet (UV) irradiation. UV facilitated the uptake of Na + into and the loss of K + from the cells (net ion flux); the effect is greatest for the shortest wavelength employed (239 m#) and is partly dependent upon the presence of oxygen. The UV effect on net ion flux persists for at least 90 minutes during which tests were made and it occurs following dosages which are without measurable effect on colony formation. The UV effect on net ion flux is decreased by acidity and prorooted by alkalinity. Addition of calcium ions in sufficient amount prevents the usual net ion flux changes observed in irradiated yeast. Increase in concentration gradient between the inside and the outside of the cell increases the net ion flux of irradiated yeast, Na + uptake leading K + loss in all cases. UV appears to act by disorganizing the constituents of the cell surface, permitting K + to leave the cell in exchange for Na +. At low intensities of UV this ionic exchange approaches equivalence, but at higher intensities more lqa + is taken up than K + is lost. Some evidence suggests that the Na + in excess over that exchanged for K + is adsorbed to charged groups produced by the photochemical effect of UV on the cell surface.

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