Dissecting a complex process

Transcription revisited: a commentary on the 1995 Cold Spring Harbor Laboratory meeting, "Mechanisms of Eukaryotic Transcription"

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Transcriptional interference caused by GCN4 overexpression reveals multiple interactions mediating transcriptional activation

Overproduction of Gcn4p in yeast cells resulted in the inhibition of transcription from promoters controlled by the GAL4 or dA:dT elements. We have demonstrated that this effect is mediated through the activation domain of Gcn4p and that the function of the transcriptional activator at the affected promoter is impaired. The inhibitory effect of Gcn4p and that the function of the transcriptional activator at the affected promoter is impaired. The inhibitory effect of Gcn4p on these promoters persisted in yeast strains disrupted for the ADA2 and/or GCN5 genes, whose products are required for only part of the transcriptional activation capacity of Gcn4p and other activators, but was alleviated by overexpression of gamma TFIIB. These results support the hypothesis that general transcription factors become unavailable at certain promoters when an activator is overexpressed and strongly imply the existence of an Ada2p/Gcn5p-independent pathway of communication between acidic activators and the basic transcription machinery. In a genetic screen, we have isolated a mutation which neutralises the squelching effects of Gcn4p. This AFR1-1 (activation function reduced) mutation is dominant, it affects the transcriptional activation properties of a number of activators and results in lethality when combined with a gcn5 disruption. Our results suggest that the AFR1 gene product is involved in the mediation of transcriptional activation.

Genetic evidence for the interaction of the yeast transcriptional co-activator proteins GCN5 and ADA2

The GCN5 and ADA2 proteins are required for the activation function of a number of transcriptional activators in the yeast Saccharomyces cerevisiae. By using appropriate LexA fusion proteins we demonstrated that both proteins are required for part of the function of the GCN4, GAL4 and the VP16 transcriptional activation domains. Analysis of a gcn5 ada2 double disruption mutant did not reveal any additive effects, suggesting that the two proteins act in the same pathway. The GCN5 and ADA2 proteins can each activate transcription when directed to the promoter region of a reporter gene, but only in the presence of a wild-type ADA2 or GCN5 gene, respectively. The activation capacity is enhanced when the corresponding endogenous gene copy is disrupted. Taken together, these genetic data suggest that the two proteins interact and define one complex that mediates transcriptional activation. The function of this complex requires the bromodomain region of the GCN5 protein.