Influence of water content on uv-induced s-mutation and killing in Serratia

Abstract

Resting cells of Serratia were spread on a dry film of gelatin on a glass slide, dried, brought to different water contents in quartz covered chambers with salt solutions giving from 33 to 97.5 per cent relative humidities (r.h.), and then uv-irradiated. After dissolving the gelatin to suspend the cells, diluting, and plating, survivors and color-sector ( s) mutants were counted. The main facts on the nature of s-mutations are reviewed. The relative uv sensitivities, measured as the slopes of the one-hit curves for killing and mutation, decreased from more than 20 to 1 when the cells were moved from 33 to 97.5 per cent r.h. and this decrease was about the same for mutation and killing. The decrease in radiosensitivity was accomplished in less than 13 min. The water content after irradiation seemed to play no appreciable role. The “protection” by water against uv cannot be due to simple dilution of supposed uv poisons, but it goes parallel with a change in the structure of the uv-absorbing DNA brought about by moisture content, which can be observed in vitro as a decrease of uv-induced gel formation by DNA. No indication of spacially “indirect” (diffusion) processes in the uv action was found. When dry (33 per cent r.h.) cells are uv-irradiated no photoreversion of either killing or mutation is observed with either dry or wet (97.5 per cent r.h.) illuminated cells. Wet uv-irradiated cells give photoreactivation of killing only when wet cells are illuminated, but s-mutations are photoreversed when wet or dry cells are exposed to light. A hypothetical reaction system which explains these effects is discussed. When cells are illuminated in the dry state, visible light alone induced killing but not s-mutation. This killing seems to be a one-hit process and its rate is the same in previously uv-irradiated and unirradiated cells. Light alone did not cause killing or mutation in cells kept in wet state; but cells previously dried and then illuminated when wet showed some killing and s-mutation induction by light alone.