Abstract. The present communication is concerned with the effects of near‐UV radiation (300–380 nm) on yeast Candida guilliermondii. It was found that certain doses of 313 nm irradiation caused inactivation of the yeast which was exhibited in a way different from the lethal action of far‐UV radiation. It was also found that the cells inactivated by 313 nm are capable of recovering vitality, if incubated for some time in a non‐nutrient medium. The yeast inactivated by far‐UV radiation also proved to be capable of recovering, though to a lesser degree. Both 334 nm radiation and non‐lethal doses at 313 nm induced the photoprotective effect against far‐UV damage. The effect was exhibited if there was a certain time interval (2–4 h) between the exposures to photoprotective light and subsequent far‐UV radiation. Within this time interval the extent of photoprotection was dependent on temperature.
The effect of red (660 nm) and far-red (730 nm) light on the stability of the yeast Candida guilliermondii to lethal U.V. radiation has been studied. Reactivation and protection were exhibited for 30 min after treatment with red light and were abolished by far-red exposure applied within this time period. The temperature dependence of the reactivation effect was also studied. The data obtained show that the properties of recovery and protection against U.V. exposure are associated with the phytochrome system of the yeast. I N T R O D U C T I O NThe reversible action of red (650 to 690 nm) and far-red (700 to 750 nm) light on the rate of reproduction of the yeast Candida guilliermondii was reported by Fraikin, Verkhoturov & Rubin (1973). The action spectra of acceleration of yeast cell division in the lag phase and the photoreversibility of this effect showed maximum values at 660 and 73onm respectively, and the effects of the intensity, dose and duration of 660 nm (R) and 730 nm (FR) light were examined. The differential spectrum FR minus R light with positive maximum at 660 nm and negative at 730 nm was recorded, as well as the spectrum of yeast fluorescence with a specific maximum of 695 nm at 650 nm light excitation. From these data, we concluded that the yeast pigment system has the basic characteristics of the phytochrome system of green plants (Hendricks, Butler & Siegelman, 1962;Briggs & Rice, 1972; Grill, 1972). In higher plants it is known that P,30 is converted in the dark to the inactive form PGe0. We investigated whether this also took place with yeast phytochrome, and determined the duration of the active form in vivo. METHODS Candida guilliermondii was grown and prepared as described by Fraikin et al. (1974).Before the inoculation of liquid medium in a shaken flask, the ceIls of the sample were exposed to monochromatic light for a short time and were then cultivated under the same conditions as the control. A 1000 W mercury lamp and a grating spectrometer with I nm mm-1 linear dispersion were used as light sources. Cell numbers were counted in a FuchsRosental chamber. The ratio of the number of cells in the experimental sample to that in the control was regarded as a measure of the effect of the light.An investigation into the effect of monochromatic light on the survival rate of yeasts exposed to U.V. radiation was carried out on wort agar at 30 "C for 24 h. For the preinoculation exposure of cells to lethal U.V. radiation at 254 nm, a type BUV-30 bactericidal lamp was used. The survival rate of yeasts was determined by counting the microcolonies.
Abstract— In experiments with the non‐photoreactivable yeast Candida guilliermondii, radiations at 313, 334 and 365 nm, having no effect on untreated cell populations, produced an ‘enhancing’ effect on the lethality of 254 nm‐pretreated cells. Wavelengths in the visible region of the spectrum did not exhibit a similar effect.
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