Evidence that complex formation by Bas1p and Bas2p (Pho2p) unmasks the activation function of Bas1p in an adenine-repressible step of ADE gene transcription.
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.
Zhang
F
F
Laboratory of Eukaryotic Gene Regulation, National Institute of Child Health and Human Development, Bethesda, Maryland 20892, USA.
Kirouac
M
M
Zhu
N
N
Hinnebusch
A G
AG
Rolfes
R J
RJ
D000225:Adenine / Q000096:biosynthesis / Q000494:pharmacology / Q000502:physiology*; D001224:Aspartic Acid; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D004271:DNA, Fungal / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018398:Homeodomain Proteins; D046911:Macromolecular Substances; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D012697:Serine Endopeptidases / Q000235:genetics / Q000378:metabolism; D015534:Trans-Activators / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D015533:Transcriptional Activation; D000099160:LexA Repressor Protein
D000225:Adenine; D001224:Aspartic Acid; D001426:Bacterial Proteins; D004271:DNA, Fungal; D005656:Fungal Proteins; D018398:Homeodomain Proteins; D046911:Macromolecular Substances; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D012697:Serine Endopeptidases; D015534:Trans-Activators; D000099160:LexA Repressor Protein; C061767:BAS1 protein, S cerevisiae; C051611:PHO2 protein, S cerevisiae