Chapter III: β-galactosidase and Thiogalactoside Transacetylase

Abstract

A complete description of the lac operon in Escherichia coli, its expression and its regulation, must include detailed knowledge of the protein products of its structural genes. These proteins are three: β-galactosidase, whose function is to catalyze the hydrolysis of lactose; the galactoside permease (or “M” protein), which is involved in transport and accumulation of lactose within the cell; and thiogalactoside transacetylase, whose function is still under investigation. It is the purpose of this chapter to summarize information on the chemistry of β-galactosidase and thiogalactoside transacetylase, and to relate such information where possible to the genetic elements of the lac operon. Much of the earlier work on β-galactosidase has been reviewed by Cohn (1957) and by Wallenfels and Malhotra (1961). The “M” protein is discussed in detail by Kennedy elsewhere in this volume.

β-GALACTOSIDASE
Isolation
β-galactosidase has been obtained in pure state from ML and K-12 strains of E. coli by fairly conventional methods of protein purification employing ammonium sulfate precipitations and DEAE cellulose or DEAE-Sephadex column procedures (Hu, Wolfe, and Reithel, 1959; Karlsson, Koorajian, Zabin, Sjostrand, and Miller, 1964; Craven, Steers, and Anfinsen, 1965). No differences have been observed between the proteins from the two sources, and it is assumed they are identical. Crystallization of the enzyme was first achieved by Wallenfels and Zarnitz (1957) by addition of ammonium sulfate to solutions of the protein in the presence of sodium chloride.

Composition
The crystalline protein was found to contain 16.1% nitrogen, 0.93% sulfur, and to contain no sugar or...