In Saccharomyces cerevisiae, AMP biosynthesis genes (ADE genes) are transcriptionally activated in the absence of extracellular purines by the Bas1p and Bas2p (Pho2p) transcription factors. We now show that expression of the ADE genes is low in mutant strains affected in the first seven steps of the pathway, while it is constitutively derepressed in mutant strains affected in later steps. Combined with epistasy studies, these results show that 5-phosphoribosyl-4-succinocarboxamide-5-aminoimidazole (SAICAR), an intermediate metabolite of the pathway, is needed for optimal activation of the ADE genes. Two-hybrid studies establish that SAICAR is required to promote interaction between Bas1p and Bas2p in vivo, while in vitro experiments suggest that the effect of SAICAR on Bas1p-Bas2p interaction could be indirect. Importantly, feedback inhibition by ATP of Ade4p, catalyzing the first step of the pathway, appears to regulate SAICAR synthesis in response to adenine availability. Consistently, both ADE4 dominant mutations and overexpression of wild-type ADE4 lead to deregulation of ADE gene expression. We conclude that efficient transcription of yeast AMP biosynthesis genes requires interaction between Bas1p and Bas2p which is promoted in the presence of a metabolic intermediate whose synthesis is controlled by feedback inhibition of Ade4p acting as the purine nucleotide sensor within the cell.In Saccharomyces cerevisiae, biosynthesis pathways are generally negatively regulated by their end product. This regulation usually occurs at two distinct levels, feedback inhibition of an enzyme of the pathway (commonly the first one) and coordinate repression at the transcriptional level of the genes encoding enzymes of the pathway.Studies on the regulation of the purine biosynthesis pathway in S. cerevisiae revealed that all the genes encoding enzymes required for AMP de novo biosynthesis are repressed at the transcriptional level by the presence of extracellular purines (adenine or hypoxanthine) (6,7,10,23). Two transcription factors, named Bas1p and Bas2p, are required for regulated activation of the ADE genes (6) as well as some histidine biosynthesis genes (2,7,35). A LexA-Bas1p fusion can activate a lexAop-lacZ reporter in the presence of Bas2p and in the absence of adenine, suggesting that the regulation process affects the interaction between the two transcription factors (44). A Bas1p subdomain, named BIRD, was identified as being critical for adenine response and Bas1p-Bas2p interaction in vivo (29). However, our understanding of how this domain senses and responds to extracellular adenine is still incomplete.Our previous work on mutants in which purine biosynthesis genes are no longer repressed by extracellular adenine allowed us to better understand the molecular nature of the signal (13). These mutations, named bra for bypass of repression by adenine, define more than nine complementation groups, several of which have been characterized. BRA7 is FCY2, the gene coding for the purine cytosine permease (Fig. 1) (13)....

The immunosuppressive drug mycophenolic acid (MPA) is a potent and specific inhibitor of IMP dehydrogenase, the first committed step of GMP synthesis. A screen for yeast genes affecting MPA sensitivity, when overexpressed, allowed us to identify two genes, IMD2 and TPO1, encoding a homologue of IMP dehydrogenase and a vacuolar pump, respectively. In parallel, 4787 yeast strains, each carrying an identified knock-out mutation, were tested for growth in the presence of MPA, allowing identification of 100 new genes affecting MPA resistance when disrupted. Disturbance of several cellular processes, such as ergosterol biosynthesis, vacuole biogenesis, or glycosylation impaired the natural capacity of yeast to resist MPA, although most of the highly sensitive mutants affected the transcription machinery (19 mutants). Expression of TPO1 and/or IMD2 was strongly affected in 16 such transcription mutants suggesting that low expression of these genes could contribute to MPA sensitivity. Interestingly, the spt3, spt8, and spt20 mutants behaved differently than other Spt-AdaGcn5-acetyltransferase (SAGA) mutants. Indeed, in these three mutants, as in previously characterized transcription elongation mutants, IMD2 expression was only affected in the presence of MPA, thus suggesting a possible role for some SAGA subunits in transcription elongation.

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