Galactokinase Activity in Streptococcus thermophilus

Authors

Robert W. Hutkins, University of Nebraska-LincolnFollow
Howard A. Morris, University of Minnesota, St. Paul, Minnesota
Larry L. McKay, University of Minnesota, St. Paul, Minnesota

Date of this Version

October 1985

Comments

Published in APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Oct. 1985, p. 777-780 Vol. 50, No. 4. Copyright 1985, American Society for Microbiology. Used by permission.

Abstract

ATP-dependent phosphorylation of [¹⁴Cigalactose by 11 strains of Streptococcus thermophilus indicated that these organisms possessed the Leloir enzyme, galactokinase (galK). Activities were 10 times higher in fully induced, galactose-fermenting (Gal⁺) strains than in galactose-nonfermenting (Gal-) strains. Lactose-grown, Gal^- cells released free galactose into the medium and were unable to utilize residual galactose or to induce galK above basal levels. Gal⁺ S. thermophilus 19258 also released galactose into the medium, but when lactose was depleted growth on galactose commenced, and galK increased from 0.025 to 0.22 μmol of galactose phosphorylated per min per mg of protein. When lactose was added to galactose-grown cells of S. thermophilus 19258, galK activity rapidly decreased. These results suggest that galK in Gal⁺ S. thermophilus is subject to an induction-repression mechanism, but that galK cannot be induced in Gal^- strains.