Genetic and biochemical characterization of the galactose gene cluster in Kluyveromyces lactis

We isolated and identified mutant strains of Kluyveromyces lactis that are defective for the Leloir pathway enzymes galactokinase, transferase, and epimerase, and we termed these loci GAL1, GAL7, and GAL10, respectively. Genetic data indicate that these three genes are tightly linked, having an apparent order of GAL7-GAL10-GAL1. This same gene order has been observed in Saccharomyces cerevisiae. Strains harboring GAL7 mutations have elevated levels of β-galactosidase, coded by an unlinked gene, galactokinase, and epimerase activities under uninduced conditions. We investigated the genetic basis of this constitutive gene expression and found no recombinants between the constitutive and Gal^- phenotypes among 76 tetrads, suggesting that either GAL7 or a tightly linked gene codes for a regulatory function. This is the second gene that has been shown to specifically coregulate expression of the genes coding for β-galactosidase and the Leloir pathway enzymes.