Classification and intragenic position of mutations in the beta-galactosidase gene of Escherichia coli.

Abstract

Following treatment with N-methyl-N′-nitro-N-nitrosoguanidine, 1,257 mutants of Escherichia coli K12 were isolated on lactose-tetrazolium medium. Of these mutants, 345 were lactose-negative and lacked appreciable β-galactosidase activity. About half of these enzyme-deficient mutants had lost the whole lactose operon; the remainder (174) were point mutations within the β-galactosidase gene. With the exception of 42 which could not be classified, the mutations were identified either as chain-terminating (UAG 57, UAA 6, UGA 60) or missense (9). There were no mutations of the reading frame and no short deletions. The unclassifiable mutants do not form crossreacting protein and are probably a type of chain-terminating mutant.