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One of the cytotoxic antimetabolites known as (N[phosphonomethyl ]-glycine is widely used as a herbicidal glyphosate. It is a target inhibitor inactivating the sixth enzyme of the shikimic pathway of the aromatic compound synthesis in plants, fungi and bacteria [1 ]. In studying the mechanisms of cytotoxic resistance to this antimetabolite in E. coli the fol­ lowing was observed. While screening the wild type cells on minimal M9 agar containing 0.5 mM gly­ phosate no spontaneous glyphosate-resistant (gly) mutants have been isolated indicating a gly r mutation frequency less than 3-Ю". However, after the treat­ ment of these cells with nitrosoguanidine at both high and low cell viability an extraordinary large number of gly mutants arise at a frequency of about 1 • 10~. This is 2 orders of magnitude higher than the level of mutability of metabolic genes [2, 3 ]. Mapping of gly r mutations showed that they are scattered around the entire chromosome [3 ]. Interestingly, half of the gly r mutants emerged at both heavy and light mutagenesis has turned out to be multiple resistant. In our experiments they could tolerate not only glyphosate but also S-2-aminoethyl-L-cysteine («Sigma», USA) (AEC) and 6-aza-uracil (AU) (synthesized by Dr. Inna Alexeeva our Institute). The frequency of the aec and aii mutation arising was also about 1-Ю"* [3]. Since the majority of E. coli Genetic Stock Center strains have been mutagenized we were interested to inquire if and how often the mutations conferring resistance to cytotoxic antimetabolites could be found as nonselected mutations among these strains. For

Among Escherichia coli Genetic Stock Center strains many are resistant to cytotoxic antimetabolites