Single Chromosomal Region Research Articles

DECREASED SURVIVAL RESULTING FROM LIQUID‐HOLDING OF U.V.‐IRRADIATED ESCHERICHIA COLI C AND SCHZZOSACCHAROMYCES POMBE

Abstract— Ultraviolet‐irradiated cells of E. coli C and of haploid wild type yeast Schizosac‐charomyces pombe, held in buffer at 22°‐25°C for various periods of time prior to plating, show a lower survival than those plated immediately after irradiation. This ‘negative liquid‐holding effect’ (NLHE) contrasts ‘liquid‐holding recovery’ (LHR), found in a number of other E. coli strains and in Saccharomyces cerevisiae. NLHE was observed at all u.v. doses tested. The effect is maximal at holding temperatures in the range 25–30°C, it is very small at 5°C and (in E. coli C) at 44°C. NLHE and LHR resemble each other in several respects. In E. coli both effects are inhibitable by the dark repair inhibitors acriflavine, caffeine and potassium cyanide. They do not occur in nutrient broth, and they are much reduced if the irradiated cells were illuminated with photoreactivating light before holding. NLHE in S. pombe shows characteristics similar to those observed in E. coli C. Mutations leading to increased u.v. sensitivity in E. coli C and S. pombe can alter the liquid‐holding response so that LHR is observed. Tetrad analysis of crosses between u.v.‐sensitive and u.v.‐resistant S. pombe strains indicates that a single chromosome region can control both u.v. sensitivity and liquid‐holding response. Several possibilities explaining NLHE are discussed. From current knowledge about dark repair processes and from the similarities between NLHE and LHR in E. coli it seems likely that the two effects reflect slight changes in the efficiency of dark repair.