The essential function of not1 lies within the Ccr4-Not complex

The five Saccharomyces cerevisiae Not proteins are associated with the Ccr4 and Caf1 proteins in 1.2 MDa and 2 MDa complexes. The Not proteins have be en proposed to repress transcription of promoters that do not contain a can onical TATA sequence, while the Ccr4 and Caf1 proteins are required for non -fermentative gene expression. The mechanism of transcriptional regulation by the Ccr4-Not complex is unknown and the role of its different components is unclear. Only Not1p is essential for yeast viability. Here, we show that most strains carrying combinations of two null alleles o f the non-essential CCR4-NOT genes are non-viable. This would suggest that the Ccr4-Not complex is essential. We find that Not1p consists of at least two domains, a C-terminal domain that is essential for yeast viability, and a N-terminal domain that is dispensable but required for yeast wild-type g rowth. The essential C-terminal domain of Not1p can associate with Not5p, a nd both proteins are present in 1.2 and 2 MDa complexes in the absence of t he N-terminal Not1p domain. In contrast, in the absence of the N-terminal d omain of Not1p, Ccr4p does not efficiently associate in large complexes nor with the C-terminal domain of Not1p. Healthy growth is observed when both domains of Not1p are expressed in trans, and is correlated with their physi cal association, together with Ccr4p, in large complexes. These results are consistent with the essential function of Not1p lying within the Ccr4-Not complex. (C) 2000 Academic Press.