Isolation of a Protein Coded for by the Permease Gene of the Lac Operon of Escherichia coli
The inducible Lac operon of Escherichia coli has been shown to possess three genetic loci: for the enzyme beta-galactosidase, for the enzyme thiogalactoside 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 has 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 , a phosphorylaccepting residue has been identified at the active centre of the membrane-linked Na+, K+ activated ATP-aise system , 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  have enabled us to develop techniques with which we have identified a cytoplasmic expression of the permease (y+) gene.
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