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The Gal4 transcription factor is a positive regulator of gene expression of galactose-induced genes . The Gal4 protein represents a large fungal family of transcription factors, Gal4 family, which includes Saccharomyces cerevisiae e.g. Oaf1, Pip2, Pdr1, Pdr3, Leu3 .

Gal4 domains
The transcription factor Gal4 include two executive domains, DNA binding and activation domains, which provide strong gene activation after galactose induction.

DNA binding domain
The N-terminal DNA binding domain belongs to the Zn(2)-C6 fungal family, which forms a Zn – cysteines thiolate cluster The Gal4 DNA Binding domain recognized specifically response element in GAL1 promoter .

Gal4 activation domain
The C-terminal activation domain belongs to the nine amino acids TransActivation Domain family, 9aaTAD, together with Oaf1, Pip2, Pdr1, Pdr3, but also p53, E2A, MLL

Regulation
Galactose induces Gal4 mediated transcription albeit Glucose causes severe repression . As a part of the Gal4 regulation, inhibitory protein Gal80 recognizes and binds to the Gal4 region (853-874 aa) including activation domain 9aaTAD . The inhibitory protein Gal80 is sequestered by regulatory protein Gal3 in Galactose dependent manner .

Mutants
The Gal4 loss-of-function mutant gal4-64 (1-852 aa, deletion of the Gal4 C-terminal 29 aa) lost both interaction with Gal80 and activation function .

In the Gal4 reverted mutant Gal4C-62 mutant , a sequence (QTAY N AFMN) with the 9aaTAD pattern emerged and restored activation function of the Gal4 protein .

Inactive constructs
The activation domain Gal4 is inhibited by C-terminal domain in some Gal4 constructs .

Proteosome
A subunit of the 26 S proteasome Sug2 regulatory protein has a molecular and functional interaction with Gal4 function . Proteolytic turnover of the Gal4 transcription factor is not required for function in vivo . The native Gal4 monoubiquitination protects from 19S-mediated destabilizing under inducing conditions .

Mediators of transcription
The Gal4 activation function is mediated by MED15 (Gal11) .

The Gal4 protein interacts also with other mediators of transcription as are Tra1, TAF9, and SAGA/MED15 complex .

Application
The broad use of the Gal4 is in yeast two hybrid system to screen or to assay protein-protein interactions in eukaryotic cells from yeast to human.

In the GAL4/UAS system, the Gal4 protein and Gal4 upstream activating region (UAS) are used to study the gene expression and function in organisms such as the fruit fly.

The Gal4 and inhibitory protein Gal80 have found application in a genetics technique for creating individually labeled homozygous cells called Mosaic analysis with a repressible cell marker MARCM.