Isolation of a Protein Coded for by the Permease Gene of the Lac Operon of Escherichia coli

Abstract

The inducible Lac operon of Escherichia coli has been shown to possess three genetic loci: for the enzyme beta-galactosidase, for the enzyme thio-galactoside transacetylase (hereafter referred to as (transacetylase), and for beta-galactoside permease, a membrane transport system [1, 2, 3]. The results of kinetic studies performed on this transport system (as in other examples of biological transport) lead to the postulate that a “carrier” substance is present, this carrier transporting the permeant through the membrane into the cytoplasm. In no case of biological transport thas the kinetic evidence for a carrier been factually substantiated by concomitant isolation of the actual carrier itself, although phospholipids have been tentatively implicated in active Na+ and K + transport in the avian salt gland [4], a phosphoryl-accepting residue has been identified at ‘the active centre of the membrane-linked Na+, K+ activated ATP-ase system [5], and both phospholipids and protein have been implicated in monosaccharide transport in the human erythrocyte [6, 7]. The facts that the Lac operon of E. coli is known (to code for only the above three components and that this operon can be induced by gratuitous substrates [8] bave enabled us to develop techniques with which we lhave identified a cytoplasmic expression of the permease (y+) gene.