Polarity of suppression in the lactose operon.

The effect of suppression on enzyme synthesis was examined in 43 amber mutations of the gene for beta-galactosidase in Escherichia coli. The ordering of mutations in the gene revealed two clear gradients in the number of molecules of suppressed beta-galactosidase formed by suppression. One gradient extended over the operator-proximal third of the gene and the other over the operator-distal third. The central third of the gene gave no consistent pattern of suppression. Assays of thiogalactoside transacetylase showed that the polarity produced by chain-terminating mutations was abolished by suppression. These experiments suggest that the polar effects of chain-terminating mutations on distal genes are the secondary results of translational defects in the mutant gene. The polarity gradients may result from a supposed secondary structure to the messenger RNA of the lactose operon.