Regulation of genes controlling synthesis of the galactose pathway enzymes in yeast.

HE genetic control of synthesis of the galactose pathway enzymes in Saccharomyces cerevisiae conforms in certain respects to the operon model proposed by JACOB and MONOD (1961) for the ,&galactosidase system of E. coli, and shown by BUTTIN (1963a, b) to be valid for the E. coli galactose system as well. Three closely linked structural genes specify the galactose pathway enzymes, galactokinase, galactose-1-phosphate-uridyl transferase (transferase), and uridine diphosphogalactose-4-epimerase (epimerase) (DOUGLAS and HAWTHORNE 1964). The three loci are under the control of an unlinked regulator gene, i, which is recognizable by its recessive mutations that permit constitutive synthesis of the three galactose enzymes (DOUGLAS and PELROY 1963). A key feature of the bacterial systems which appears to be absent in the yeast system in the close association of an operator gene with the structural genes (DOUGLAS and HAWTHORNE 1964). The operator locus was defined originally as a region linked to the structural genes in which two types of mutations occurred: O", which were expressed as cis-dominant for constitutive synthesis of the operon proteins, and Oo, which prevented synthesis of all of the proteins of the operon. JACOB and MONOD (1965) have redefined the operator locus in the p-galactosidase system of E. coli as the site of repressor recognition identified by 0" mutations. The 0" mutations in this system are now considered to be polarity mutations within the first structural gene of the operon (BECKWITH 1964). Mutations in yeast that result in failure to synthesize the three galactose enzymes and thus resemble phenotypically mutations of the Oo type can be readily isolated. However, these are not polarity mutants nor are they mutants in which inducer uptake or metabolism is defective. They map in the GA, locus which segregates independently of the galactose structural genes and their phenotype is unchanged in combination with i(DOUGLAS and HAWTHORNE 1964). The occurrence of mutations in the yeast galactose system which display the dominant constitutive phenotype and the relationship of these mutants to the GA, region and to the structural genes for the galactose enzymes is the subject of the present paper.