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 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 [4], a phosphorylaccepting residue has been identified at the active centre of the membrane-linked Na⁺, K⁺ activated ATP-aise 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] have enabled us to develop techniques with which we have identified a cytoplasmic expression of the permease (y⁺) gene.

The major items of equipment used in this study were provided from a grant to Prof. A. J. Birch of this department by the Nuffield Foundation.