Research Article 12: Some Properties of Insertion Mutations in the lac Operon

Abstract

We have described the isolation and properties of spontaneous mutations in the lactose operon (Malamy, 1966). Using the fact that cells deficient in the enzyme UDPGal 4-epimerase are caused to lyse when growing in the presence of galactose or lactose, we determined plating conditions which only allowed the survival of lac* non-fermenting mutants (as well as secondary mutations in the gal* system) arising spontaneously in the population. Several different types of mutants were isolated including complete and partial deletions of the lac genes. However, reverting mutations mapping throughout the z and y genes were predominant. We have emphasized the unique polarity properties of certain of these reverting mutations mapping in the ω region of the z gene (Ullmann, Perrin, Jacob, and Monod, 1965). Fourteen out of eighteen strains examined, mapping at many sites within the ω segment (Fig. 1), exhibit a complete polar effect in contrast to all other reverting mutations in this segment, nonsense and missense, (Newton, Beckwith, Zipser, and Brenner, 1965; Perrin, 1963). The complete polar spontaneous mutants fail to synthesize any detectable permease or transacetylase activity (Malamy, 1966). Other studies with these complete polar mutants indicate that they possess a functioning i gene (Malamy, unpubl.) and are able to synthesize almost normal amounts of the early β-galactosidase peptides α and β (Morrison and Malamy, unpubl.).

On the basis of mutagen-induced reversions and the lack of response to nonsense suppressors, it was concluded that the complete polar mutations in the ω segment were of the frameshift class and...