Suppressors of frameshift mutations in Salmonella typhimurium

Suppressors specific for frameshift mutations have been studied. Of 21 frameshift mutations of the histidine operon selected for study, 13 gave rise to ICR^† † Abbreviations used: ICR-191, 2-chloro-6-methoxy-9-(3-(2-chloroethyl) aminopropylamino) acridine dihydrochloride; DES, diethylsulfate; 2-AP, 2-aminopurine nitrate; NG, N-methyl-N′-nitro-N-nitrosoguanidine; INT, 2-p-iodophenyl-3-p-nitrophenyl-5-phenyltetrazolium chloride.-induced external suppressors. Five of these mutants also gave rise to nitrosoguanidine-induced suppressors. Two frameshift mutations in the tryptophan operon were found which could be suppressed externally along with certain his frameshift mutations. Most of the externally suppressible mutations ( 12 15) are those tentatively classified as (+1) frameshifts. Forty-eight independent suppressor mutations were placed in 9 groups on the basis of their ability to suppress 13 his frameshift mutations. The 13 suppressible mutations are apparently of two general types. No cross-suppression of mutations of these two types has been observed. The efficiency of suppression was estimated for at least one suppressor from each of 8 groups. The efficiency of suppression ranges from 1% to about 15%.