Forward mutation assays using bacteria and 8-azaguanine

Following an idea suggested by Thilly (1977, personal communication), we investigated the possibility of screening chemical mutagens by using forward mutation in bacteria employing 8-azaguanine, a genetic marker widely used in eukaryotes. Tests were made on several strains of Salmonella typhimurium obtained from Ames, permeable to many chemical agents and without an excision repair system. We obtained forward and reverse mutation in liquid medium with the following eight compounds which cause missense or frameshift mutations in the set of Ames' bacteria: N-methyl- N′-nitro- N-nitrosoguanidine, ethyl methanesulfonate, methyl methanesulfonate, β-propiolactone, streptozotocin, hycanthone, 9-aminoacridine, and 2-nitrofluorene. When the increases in the mutation frequencies induced by the test compounds were calculated, it was found that using only one strain of Salmonella typhimurium (TA 1535) it was possible to demonstrate the forward mutagenic activity in all eight compounds tested. Reverse mutation was often more sensitive than forward mutation, when the strains sensitive to the particular mutagen were used. Selection for histidine independence, on the other hand, requires at least three strains of Salmonella typhimurium to show the mutagenic activity of the eight chemicals. All the compounds were found to be highly mutagenic in the Ames' agar test.