The entire DNA sequence of chromosome III of the yeast Saccharomyces cerevisiae has been determined. This is the first complete sequence analysis of an entire chromosome from any organism. The 315-kilobase sequence reveals 182 open reading frames for proteins longer than 100 amino acids, of which 37 correspond to known genes and 29 more show some similarity to sequences in databases. Of 55 new open reading frames analysed by gene disruption, three are essential genes; of 42 non-essential genes that were tested, 14 show some discernible effect on phenotype and the remaining 28 have no overt function.

When yeast cells are grown under conditions of amino acid limitation, transcription of amino acid biosynthetic genes is increased through the action of the GCN4 transcriptional regulator. gcnS mutant strains exhibit poor growth under such conditions. We have established that GCN4 requires the function of GCN5 in order to promote normal levels of transcriptional activation. In addition, we have shown that GCN5 is also required for the activity of the HAP2-HAP3-HAP4 transcriptional activation complex, which mediates the transcription of genes involved in respiratory functions. Thus, GCN5 is a new member of the recently revealed general class of transcriptional regulators that collaborate with certain specific DNA binding activators to promote high levels of transcription. We have cloned and sequenced the GCNS gene. The deduced GCN5 protein contains a region conserved in other yeast, Drosophila and human proteins, all members of this new class of transcriptional activators.

The access of transcription factors to eukaryotic promoters often requires modification of their chromatin structure, which is accomplished by the action of two general classes of multiprotein complexes. One class contains histone acetyltransferases (HATs), such as Gcn5 in the SAGA complex, which acetylate nucleosomal histones. The second class contains ATPases, such as Swi2 in the Swi/Snf complex, which provide the energy for nucleosome remodelling. In several promoters these two complexes cooperate but their functional linkage is unknown. A protein module that is present in all nuclear HATs, the bromodomain, could provide such a link. The recently reported in vitro binding of a HAT bromodomain with acetylated lysines within H3 and H4 amino-terminal peptides indicates that this interaction may constitute a targeting step for events that follow histone acetylation. Here we use a suitable promoter to show that bromodomain residues essential for acetyl-lysine binding are not required in vivo for Gcn5-mediated histone acetylation but are fundamental for the subsequent Swi2-dependent nucleosome remodelling and consequent transcriptional activation. We show that the Gcn5 bromodomain stabilizes the Swi/Snf complex on this promoter.

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Overexpression of the YAP1 transcriptional activator renders yeast cells resistant to multiple metabolic inhibitors. In an effort to identify other gene products required for this phenotype we have isolated genomic mutations which neutralize this effect. One such mutation was further characterized and the affected gene was shown to be identical to TPS2 which encodes trehalose phosphate phosphatase, an enzyme catalysing the second step in trehalose biosynthesis. We have analysed the transcriptional regulation of the TPS2 gene and have shown that its transcription is induced by a variety of stressful conditions caused by metabolic inhibitors, osmotic shock and heat shock. This transcriptional activation is mediated by multiple stress promoter elements (C4T) and requires the function of Yap1p as well as reduced activity of the cAMP‐regulated protein kinase. Using an appropriate reporter gene we have shown that Yap1p is generally required for transcriptional regulation through the C4T stress element. These results show that the YAP1 protein has a pivotal role in the metabolic stress response and the acquisition of stress tolerance.