β -Galactosidase, the Lactose Permease Protein, and Thiogalactoside Transacetylase

Abstract

INTRODUCTION The three structural genes of the lac operon, Z , Y , and A , code for β -galactosidase, the lactose permease protein, and thiogalactoside transacetylase, respectively. Since the discussion of these gene products in The Lactose Operon (Zabin and Fowler 1970; Kennedy 1970), a considerable amount of work has been carried out, particularly on β -galactosidase. In 1970 the subunit structure of this enzyme was known with reasonable certainty, but some controversy remained. Now the primary structure has been completed. Much new information is available on β -galactosidase from strains with mutations in the lacZ gene, on complementation, and on immunological properties. Interesting fusion proteins have been described. Considerably less information has been accumulated on the lacY gene product, and in fact, the lactose permease protein has not yet been obtained in chemically pure form. The function of thiogalactoside transacetylase has been a mystery for a long time. Recently evidence has been presented for a possible physiological role in the cell. A detailed study of its enzymatic characteristics has been carried out, and a beginning has been made toward determining the amino acid sequence of this protein. Such information on the three proteins is presented here and is discussed primarily from a biochemical point of view. β -GALACTOSIDASE Production and Isolation β -Galactosidase ( β -D-galactoside galactohydrolase E. C. 3.2.1.23) accounts for up to 5% of the total protein in haploid strains of Escherichia coli. Considerably higher levels can be obtained from certain partial diploids (Fowler 1972a). When the strain A324–5 was grown on minimal medium containing...