GAL4 encodes the archetypal member of the Zn(II)2Cys6 family, also called the zinc cluster family, of transcriptional regulators; there are approximately 46 proteins of this family in S. cerevisiae (reviewed in 5 and 6). Gal4p mediates expression of most of the genes involved in utilization of galactose, a relatively poor carbon source that is used only if glucose or other carbon sources are not available. Galactose induces expression of the Gal4p-regulated genes, which include the galactose utilization genes GAL1, GAL2, GAL3, GAL7, GAL10, and GAL80 as well as GCY1, LAP3, MTH1, PCL10, REE1, and FUR4 (7, 8, 9). Gal4p-regulated genes contain upstream activating sequences (UASGAL) in their promoter regions to which Gal4p binds; when galactose is present as the sole carbon source, bound Gal4p recruits the transcription machinery to actively transcribe these genes (10). The number of UASGAL sites in the promoters of the genes varies, which plays a role in the extent of activation possible; induction of GAL1, GAL2, GAL7, and GAL10 is over 1000-fold (10, reviewed in 11).

When cells are grown in glucose as carbon source, GAL4 transcription is reduced roughly 5-fold, a modest decrease that nevertheless has a large repressive effect on transcription of genes under Gal4p control (12). Growth in the presence of other carbon sources, such as raffinose, allows production of Gal4p, but its activity is inhibited through interaction with Gal80p, which blocks the surface of Gal4p that interacts with the transcription machinery; this Gal4p-Gal80p complex is tethered to upstream activating sequences of the GAL genes through the DNA binding region of Gal4p (reviewed in 13 and 11). When cells are fed galactose, the galactose sensor protein Gal3p enters the nucleus and binds to Gal80p (14). As the Gal3p-Gal80p complex (but not uncomplexed Gal80p) can be identified in both the cytoplasm and the nucleus, the Gal3p-Gal80p interaction may help prevent further interaction between Gal80p and Gal4p (14). Active Gal4p recruits the SAGA complex, Mediator complex, and other components of the transcriptional machinery to initiate transcription (15, 16, 17).

Activation by Gal4p relies on an acidic C-terminal region that both stimulates transcription and also binds Gal80p (18, 19, 20). In addition to its interactions with the transcription machinery, this acidic activating domain (AD) interacts with Sug1p and Sug2p, subunits of the 19S regulatory particle of the 26S proteasome (reviewed in 13), and is a site of monoubiquitylation (21, 22). Gal4p binds the UASGAL site as a dimer, and both DNA binding and dimerization are mediated by sequences in the N-terminal region (23). Gal4p also undergoes phosphorylation during activation, although studies indicate this phosphorylation is not essential for activity (13). The DNA binding domain of Gal4p can be physically separated from its transcription activation domain; thus these domains have been used in the development of tools, such as the two-hybrid assay, for studies of transcription regulation and protein-protein interactions (reviewed in 24).

While proteins containing Zn(II)2Cys6 motifs are found in S. cerevisiae and other fungi, they have not been identified in prokaryotes or higher eukaryotes (reviewed in 6).

Last updated: 2010-12-01