GAL3 encodes a transcriptional regulator involved in gene activation in response to galactose (6). Among the genes that, in conjunction with Gal4p and Gal80p, are regulated by Gal3p are the GAL metabolic genes GAL1, GAL10, GAL2, and GAL7 (reviewed in 7). In the presence of galactose and ATP, Gal3p sequesters the transcriptional repressor Gal80p in the cytoplasm which relieves inhibition of the Gal4p transcription factor and results in GAL gene expression (4, 8). GAL3 transcription is also regulated by the Gal3p/Gal80p/Gal4p feedback mechanism that regulates the other GAL genes. However, unlike expression of the GAL structural genes, which is tightly controlled because of the presence of multiple upstream activation sites (UAS-GALs) in their promoters, GAL3 has high basal levels of transcription due to the presence of only a single UAS-GAL in its promoter (9).

GAL3 is thought to have arisen from GAL1 as a result of the S. cerevisiae whole genome duplication and the two proteins share 90% amino acid similarity (10). Insertion of just two amino acids (a serine and an alanine) directly after amino acid 164 is able to confer galactokinase activity on Gal3p. Null mutants of gal3 exhibit at 3-5 day lag in transcriptional induction (as opposed to several minutes in wild-type cells), a phenotype which can be complemented by GAL1 overexpression (11).

Last updated: 2006-10-04