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Originally published as Genetics Published Articles Ahead of Print on February 3, 2008.

Genetics, Vol. 178, 725-736, February 2008, Copyright © 2008
doi:10.1534/genetics.107.074799

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Genetic Evidence for Sites of Interaction Between the Gal3 and Gal80 Proteins of the Saccharomyces cerevisiae GAL Gene Switch

Cuong Q. Diep1, Xiaorong Tao2, Vepkhia Pilauri3, Mandy Losiewicz, T. Eric Blank4 and James E. Hopper5

Department of Biochemistry and Molecular Biology, Pennsylvania State University, College of Medicine, Hershey, Pennsylvania 17033

5 Corresponding author: Ohio State University, Department of Biochemistry, Room 233, Biological Sciences Bldg., 484 W. 12th Ave., Columbus, OH 43210.
E-mail: hopper.65{at}osu.edu

Galactose-activated transcription of the Saccharomyces cerevisiae GAL genes occurs when Gal3 binds the Gal4 inhibitor, Gal80. Noninteracting variants of Gal3 or Gal80 render the GAL genes noninducible. To identify the binding determinants for Gal3's interaction with Gal80 we carried out GAL3–GAL80 intergenic suppression analyses and selected for new GAL3 mutations that impair the Gal3–Gal80 interaction. We show that a GAL3C-D368V mutation can suppress the noninducibility due to a GAL80S-1-G323R mutation, and a GAL80-M350C mutation can suppress the noninducibility due to a gal3-D111C mutation. A reverse two-hybrid selection for GAL3 mutations that impair the Gal3–Gal80 interaction yielded 12 single-amino-acid substitutions at residues that are predicted to be surface exposed on Gal3. The majority of the affected Gal3 residues localized to a composite surface that includes D111 and a sequence motif containing D368, which has been implicated in interaction with Gal80. The striking colocalization of intergenic suppressor residues and Gal80 nonbinder residues identifies a Gal3 surface that likely interacts with Gal80.






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