Mutation Research, 202 (1988) 71-75 71 Elsevier MTR 04637 Influence of nalidixic acid on killing and mutation of V79 cells exposed to different damaging agents Rita Ghosh (Datta) and Sukhendu B. Bhattacharjee Crystallography and Molecular Biology Division, Saha Institute of Nuclear Physics, Calcutta-700 064 (India) (Received 15 December 1987) Revision received 15 March 1988) (Accepted 15 March 1988) Keywords: Nalidixic acid; V79 cells; DNA topolsomerase, inhibitor; DNA damaging agents; N-Methyl-N'-nitro-N-nitrosoguanidine Summary The effect of treatment with nalidixic acid, an inhibitor of DNA topoisomerase, after exposure of V79 cells to different DNA-damaging agents on the induction of killing and mutation has been studied. The DNA-damaging agents were ultraviolet light, y-rays and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). It was seen that treatment with nalidixic acid potentiated the killing by MNNG and suppressed the induction of mutation. However, it had no influence upon killing and mutation by UV light and y-rays. The difference in the observed results could be due to the nature of the damage induced and its repair in relation to the function of topoisomerases. The eukaryotic genome exists in a condensed supertwisted form; relaxation of this structure is needed to make such DNA more accessible for its various functions. Topoisomerases are a class of enzyme that control the topological states of the DNA molecule. These enzymes have been im- plicated in many nuclear processes such as repli- cation, recombination, transcription (Gellert, 1981; Wang, 1985) and chromatin assembly. Using in- hibitors of topoisomerases (novobiocin and nali- dixic acid), several authors have shown that these enzymes could have a role in the repair of DNA damaged by different agents (Mattern and Scudeiro, 1981; Cleaver, 1982; Collins et al., 1982; Correspondence: Dr. S.B. Bhattacharjee, Professor and Head, Crystallography and Molecular Biology Division, Saha In- stitute of Nuclear Physics, I/AF Salt Lake, Calcutta-700064 (India). Mattern et al., 1982, 1983). However, no work has been done on the mutagenic aspects of such treat- ment. We were, therefore, interested in observing the roles played by nalidixic acid in the killing and mutation of V79 cells by different types of damag- ing agents such as N-methyl-N'-nitro-N-nitro- soguanidine (MNNG), ultraviolet (UV) light and y-rays. Materials and methods Cell culture The cell line used was V79 Chinese hamster cells. Components of growth medium (MEM) and MNNG, benzamide, nalidixic acid, neomycin sulphate and 6-thioguanine were purchased from Sigma Chemical Company, penicillin and strep- tomycin from local sources. Medium was supple- mented with 10% dialysed goat serum (Bhat- tacharjee and Pal, 1982). 0027-5107/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)