Polarity of amber mutations and suppressed amber mutations in the galactose operon of E. coli

Summary Amber mutants in the t gene of the galactose operon have been examined for polarity in the presence and absence of the suppressors su_I and su_yMel.In the absence of suppressors there is a gradient of polarity with the more polar mutations nearer the epimerase gene. This polarity is cis-dominant. Amber t mutants have raised epimerase levels but this effect is recessive. The operon is normally inducible in the presence of amber mutations. A double amber mutant had the polarity of the mutation nearest the epimerase end of the gene.In the presence of suppressors there is practically no gradient of polarity. This is in disagreement with the model proposed by Martin et al. (1966) and Yanofsky and Ito (1966). Modifications of this model to fit the present data are proposed.