GENETIC AND MOLECULAR ANALYSIS OF THE GAL3 GENE IN THE EXPRESSION OF THE GALACTOSE/MELIBIOSE REGULON OF SACCHAROMYCES CEREVISIAE

Abstract

During the galactose adaptation period of a Saccharomyces cerevisiae strain bearing a naturally occurring gal3 allele, we found a longer induction lag and slower rate of accumulation of GAL10 and MEL1 RNAs compared to wild-type strains. A strain of genotype gal3 gal1 gal7 is noninducible for MEL1 gene expression, but this expression block is bypassed by overexpression of the GAL4 gene or by deletion of the GAL80 gene, either of which causes a constitutive phenotype. An otherwise wild-type strain that bears a chromosomal gal3 gene disruption mutation does not produce wild-type GAL3 RNA and exhibits induction comparable to a strain bearing the naturally occurring gal3. Based on this array of results, we conclude that the GAL3 gene product executes its function at a point before GAL4 mediated transcription of the GAL1-10-7 and MEL1 genes. Thus, the data are consistent with the previously advanced hypothesis that the GAL3 gene product functions to synthesize the inducer or coinducer molecule. In experiments in which the presence of either the plasmid-carried cloned GAL3 gene or the plasmid-carried cloned GAL1-10-7 genes allows MEL1 induction of a gal3 gal1 gal7 cell, we find that loss of the plasmid results in the shutoff of MEL1 expression even when galactose is continuously present. Either GAL3 function or GAL1-10-7 functions are therefore required for both the initiation and the maintenance of the induced state. Since the strains bearing either the naturally occurring gal3 allele or the gal3 disruption (null) allele do induce, the plasmid loss experiments indicate the existence of two completely independent induction initiation-maintenance pathways, one requiring GAL3 function, the other requiring GAL1-10-7 function. Finally, Northern blot analysis reveals two major GAL3 transcripts that differ in size by approximately 500 nucleotides.

Public good‐driven release of heterogeneous resources leads to genotypic diversification of an isogenic yeast population

“…The resulting monosaccharides each have antagonistic regulatory effect on MEL1. Although galactose acts as an inducer of the system, presence of glucose represses the expression of GAL/MEL pathway even in presence of galactose via the known catabolite repression pathway ( Adams 1972 ; Kew and Douglas 1976 ; Johnston and Hopper 1982 ; Post-Beittenmiller et al 1984 ; Torchia and Hopper 1986 ). MEL1 has an Upstream Activating Sequence (UAS) for induction by Gal4p in presence of galactose and an Upstream Repressible Sequences (URS) for glucose repression mediated by Mig1p (reviewed in Lohr et al 1995 ; Rubio-Texeira 2005 ).…”

Yeast galactokinase in closed conformation can switch between catalytic and signal transducer states

“…We introduced K266R and Y441A substitutions using SDM and integrated the GAL1 K266R and GAL1 Y441A alleles at the GAL1 locus of Sc723 gal3Δ GAL1 wt (see material and methods for details) to generate Sc723gal3ΔGAL1 Y441A and Sc723 gal3ΔGAL1 K266R strains. It is reported a gal3Δ strain shows long term adaptation phenotype [27, 34, 55]. Long term adaptation phenotype can be demonstrated by spotting assay [56, 57].…”

“…On the other hand Sc Gal1p was able to retain both its enzymatic as well as signal transduction activity. Thus, in a wild type S.cerevisaie strain, once the transcriptional induction of GAL genes is ensued by galactose, the induced state can be sustained even after the withdrawal of Sc Gal3p function, as long as galactose is available [34, 35]. In a wild type strain, once the induction occurs, the maintenance of the induced state is sustained by Sc Gal1p but not Sc Gal3p [36].…”

Galactokinase Encoded by GAL1 Is a Bifunctional Protein Required for Induction of the GAL Genes in Kluyveromyces lactis and Is Able To Suppress the gal3 Phenotype in Saccharomyces cerevisiae

“…The genes in S. cerevisiae encoding enzymes of galactose and melibiose metabolism are induced shortly after a cell grown on a nonfermentable carbon source receives galactose (1,6,33). The induction mechanism is dependent on the function of GAL4, GAL80, and GAL3 (reviewed in reference 13).…”

“…GAL4 and GAL80 encode proteins which are directly involved in transcription activation and repression of galactose-governed promoters (13). The GAL3 gene has been recently analyzed in detail, but its function remains unknown (2,33). Most noticeably, GAL3 mutants exhibit an induction time increased by several days (31,33,38), leading to the hypothesis that GAL3 is responsible for the synthesis of the inducer molecule (6).…”