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Mol. Cell. Biol., 06 1997, 3272-3283, Vol 17, No. 6
F Zhang, M Kirouac, N Zhu, AG Hinnebusch and RJ Rolfes
Bas1p and Bas2p (Pho2p) are Myb-related and homeodomain DNA binding
proteins, respectively, required for transcription of adenine biosynthetic
genes in Saccharomyces cerevisiae. The repression of ADE genes in
adenine-replete cells involves down-regulation of the functions of one or
both of these activator proteins. A LexA-Bas2p fusion protein was found to
activate transcription from a lexAop-lacZ reporter independently of both
BAS1 function and the adenine levels in the medium. In contrast, a
LexA-Bas1p fusion activated the lexAop reporter in a BAS2-dependent and
adenine-regulated fashion. The DNA binding activity of Bas2p was not needed
for its ability to support activation of the lexAop reporter by LexA-Bas1p,
indicating that LexA- Bas1p recruits Bas2p to this promoter. The activation
functions of both authentic Bas1p and LexA-Bas1p were stimulated under
adenine-repressing conditions by overexpression of Bas2p, suggesting that
complex formation by these proteins is inhibited in adenine-replete cells.
Replacement of Asp-617 with Asn in Bas1p or LexA-Bas1p allowed either
protein to activate transcription under repressing conditions in a manner
fully dependent on Bas2p, suggesting that this mutation reduces the
negative effect of adenine on complex formation by Bas1p and Bas2p.
Deletions of N-terminal and C-terminal segments from the Bas1p moiety of
LexA-Bas1p allowed high-level activation by the truncated proteins
independently of Bas2p and adenine levels in the medium. From these results
we propose that complex formation between Bas1p and Bas2p unmasks a latent
activation function in Bas1p as a critical adenine- regulated step in
transcription of the ADE genes.
Copyright © 1997, American Society for Microbiology
Evidence that complex formation by Bas1p and Bas2p (Pho2p) unmasks the activation function of Bas1p in an adenine-repressible step of ADE gene transcription
Laboratory of Eukaryotic Gene Regulation, National Institute of Child Health and Human Development, Bethesda, Maryland 20892, USA.
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