RE-INITIATION OF POLYPEPTIDE SYNTHESIS AND POLARITY IN LAC OPERON OF ESCHERICHIA COLI

Strains of Escherichia coli which contain two nonsense mutations of either the z and y genes or the z gene of the lac operon have been constructed and examined for levels of transacetylase activity. Comparison of the polarity of these double mutants to the polarity of the parent strains, which contain only one of the mutations, supports the following conclusions. (a) Expression of transacetylase activity by z- nonsense mutants depends upon translation of the y message. (b) Premature chain termination at nonsense triplets in the part of z closest to y (operator-distal region) is followed by re-initiation of polypeptide synthesis predominately at the start sequence for the y message. (c) Premature chain termination at nonsense triplets which map in the middle and first (operator-proximal) part of z is followed by re-initiation of polypeptide synthesis at sites within the z gene message. This interpretation is supported in part by the finding that nonsense mutants of the middle section of z produce a polypeptide corresponding to the C-terminal end of β-galactosidase. The nature of the re-initiation sites in z and their relationship to the polarity of z- nonsense mutants are discussed. © 1969.