Domains of p85cdc10 required for function of the fission yeast DSC-1 factor

Repression of G1/S transcription is mediated via interaction of the GTB motifs of Nrm1 and Whi5 with Swi6

In Saccharomyces cerevisiae, G1/S transcription factors MBF and SBF regulate a large family of genes important for entry to the cell cycle and DNA replication and repair. Their regulation is crucial for cell viability, and it is conserved throughout evolution. MBF and SBF consist of a common component, Swi6, and a DNA-specific binding protein, Mbp1 and Swi4, respectively. Transcriptional repressors bind to and regulate the activity of both transcription factors. Whi5 binds to SBF and represses its activity at the beginning of the G1 phase to prevent early activation. Nrm1 binds to MBF to repress transcription as cells progress through S phase. Here, we describe a protein motif, the GTB motif (for G1/S transcription factor binding), in Nrm1 and Whi5 that is required to bind to the transcription factors. We also identify a region of the carboxy terminus of Swi6 that is required for Nrm1 and Whi5 binding to their target transcription factors and show that mutation of this region overrides the repression of MBF- and SBF-regulated genes by Nrm1 and Whi5. Finally, we show that the GTB motif is the core of a functional module that is necessary and sufficient for targeting of the transcription factors by their cognate repressors.

Deconvolution of chromatin immunoprecipitation-microarray (ChIP-chip) analysis of MBF occupancies reveals the temporal recruitment of Rep2 at the MBF target genes

MBF (or DSC1) is known to regulate transcription of a set of G(1)/S-phase genes encoding proteins involved in regulation of DNA replication. Previous studies have shown that MBF binds not only the promoter of G(1)/S-phase genes, but also the constitutive genes; however, it was unclear if the MBF bindings at the G(1)/S-phase and constitutive genes were mechanistically distinguishable. Here, we report a chromatin immunoprecipitation-microarray (ChIP-chip) analysis of MBF binding in the Schizosaccharomyces pombe genome using high-resolution genome tiling microarrays. ChIP-chip analysis indicates that the majority of the MBF occupancies are located at the intragenic regions. Deconvolution analysis using Rpb1 ChIP-chip results distinguishes the Cdc10 bindings at the Rpb1-poor loci (promoters) from those at the Rpb1-rich loci (intragenic sequences). Importantly, Res1 binding at the Rpb1-poor loci, but not at the Rpb1-rich loci, is dependent on the Cdc10 function, suggesting a distinct binding mechanism. Most Cdc10 promoter bindings at the Rpb1-poor loci are associated with the G(1)/S-phase genes. While Res1 or Res2 is found at both the Cdc10 promoter and intragenic binding sites, Rep2 appears to be absent at the Cdc10 promoter binding sites but present at the intragenic sites. Time course ChIP-chip analysis demonstrates that Rep2 is temporally accumulated at the coding region of the MBF target genes, resembling the RNAP-II occupancies. Taken together, our results show that deconvolution analysis of Cdc10 occupancies refines the functional subset of genomic binding sites. We propose that the MBF activator Rep2 plays a role in mediating the cell cycle-specific transcription through the recruitment of RNAP-II to the MBF-bound G(1)/S-phase genes.

Periodic accumulation of cdc15 mRNA is not necessary for septation in Schizosaccharomyces pombe

Analysis of Schizosaccharomyces pombe mutants that are defective in septum formation and cytokinesis has identified the product of the cdc15 gene as a key element in formation of a division septum. S. pombe cells lacking cdc15p function cannot assemble a functional medial ring, and do not make a division septum. cdc15 mRNA accumulates periodically during the cell cycle, peaking after entry into mitosis, and increased expression of the gene in G2-arrested cells can promote F-actin ring formation. Here, we have investigated the effects of mutations that block cell division upon the expression of cdc15 in synchronised cell populations, and analysed the expression of cdc15 when septum formation is induced by ectopic activation of the septation signalling network. We concluded the following: (i) the septation signalling network genes are not required for periodic accumulation of cdc15 mRNA; (ii) induction of septum formation in G2-arrested cells by activation of the septation signalling network does not result in accumulation of cdc15 mRNA, which is therefore not a prerequisite for septum formation; (iii) failure to turn off septum formation at the end of mitosis results in continued expression of cdc15; and (iv) periodic accumulation of cdc15 mRNA is mediated by a 97 bp region 5' to the mRNA start site.

Cell cycle-regulated transcription in fission yeast: Cdc10-Res protein interactions during the cell cycle and domains required for regulated transcription

In Schizosaccharomyces pombe the MBF (DSC1) complex mediates transcriptional activation at Start and is composed of a common subunit called Cdc10 in combination with two alternative DNA-binding partners, Res1 and Res2. It has been suggested that a high-activity MBF complex (at G1/S) is switched to a low-activity complex (in G2) by the incorporation of the negative regulatory subunit Res2. We have analyzed MBF protein-protein interactions and find that both Res proteins are associated with Cdc10 throughout the cell cycle, arguing against this model. Furthermore we demonstrate that Res2 is capable of interacting with a mutant form of Cdc10 that has high transcriptional activity. It has been shown previously that both Res proteins are required for periodic cell cycle-regulated transcription. Therefore a series of Res1-Res2 hybrid molecules was used to determine the domains that are specifically required to regulate periodic transcription. In Res2 the nature of the C-terminal region is critical, and in both Res1 and Res2, a domain overlapping the N-terminal ankyrin repeat and a recently identified activation domain is important for mediating cell cycle-regulated transcription.

Structural and functional architecture of the yeast cell-cycle transcription factor swi6

The structural and functional organisation of Swi6, a transcriptional regulator of the budding yeast cell cycle has been analysed by a combination of biochemical, biophysical and genetic methods. Limited proteolysis indicates the presence of a approximately 15 kDa N-terminal domain which is dispensable for Swi6 activity in vivo and which is separated from the rest of the molecule by an extended linker of at least 43 residues. Within the central region, a 141 residue segment that is capable of transcriptional activation encompasses a structural domain of approximately 85 residues. In turn, this is tightly associated with an adjacent 28 kDa domain containing at least four ankyrin-repeat (ANK) motifs. A second protease sensitive region connects the ANK domain to the remaining 30 kDa C-terminal portion of Swi6 which contains a second transcriptional activator and sequences required for heteromerisation with Swi4 or Mbp1. Transactivation by the activating regions of Swi6 is antagonised when either are combined with the central ankyrin repeat motifs. Hydrodynamic measurements indicate that an N-terminal 62 kDa fragment comprising the first three domains is monomeric in solution and exhibits an unusually high frictional coefficient consistent with the extended, multi-domain structure suggested by proteolytic analysis.

Control of S-phase periodic transcription in the fission yeast mitotic cycle

In fission yeast, passage through START and into S-phase requires cyclin-dependent kinase (CDK) activity and the periodic transcription of genes essential for S-phase ('S-phase transcription'). Here we investigate the control of this transcription in the mitotic cell cycle. We demonstrate that the periodicity of S-phase transcription is likely to be controlled independently of CDK activity. This contrasts with the equivalent system in budding yeast. Furthermore, the CDK function required for S-phase acts after the onset of S-phase transcription and after the accumulation of cdc18p, a critical target of this transcriptional machinery. We investigate the role of individual components of the S-phase transcriptional machinery, cdc10p, res1p, res2p and rep2p, and define a new role for res2p, previously demonstrated to be important in the meiotic cycle, in switching off S-phase transcription during G2 of the mitotic cycle. We show that the presence of the in vitro bandshift activity DSC1, conventionally thought to represent the active complex, requires res2p and correlates with inactive transcription. We suggest that S-phase transcription is controlled by both activation and repression, and that res2p represses transcription in G2 of the cell cycle as a part of the DSC1 complex.

Positive and negative roles for cdc10 in cell cycle gene expression

In this paper we describe properties of the cdc10-C4 mutant of the fission yeast Schizosaccharomyces pombe. The cdc10+ gene encodes a component of the DSC1Sp/MBF transcription complex, which is required for cell-cycle regulated expression at G1-S of several genes via cis-acting MCB (MIuI cell cycle box) elements. At permissive temperatures cdc10-C4 causes expression of MCB-regulated genes through the whole cell cycle, which in asynchronously dividing cells is manifested in overall higher expression levels. This overexpression phenotype is cold sensitive: in cdc10-C4 cells, MCB genes are expressed offprogressively higher levels at lower temperatures. In heterozygous cdc10-C4/cdc10+ diploid strains, MCB-regulated genes are not overexpressed, suggesting that loss, rather than alteration, of function of the cdc10-C4 gene product is the reason for unregulated target gene expression. Consistent with this, the cdc10-C4 mutant allele is known to encode a truncated protein. We have also overexpressed the region of the cdc10 protein absent in cdc10-C4 under the control of an inducible promoter. This induces a G1 delay, and additionally causes a reduction of the overexpression of MCB genes in cdc10-C4 strains. These results suggest that DSC1Sp/MBF represses, as well as activates, MCB gene expression during the cell cycle.

Cell cycle regulated transcription in yeast

At least four different classes of cell cycle regulated gene exist in yeast: G1 cyclins and DNA synthesis genes are expressed in late G1; histone genes in S phase; genes for transcription factors, cell cycle regulators and replication initiation proteins in G2; and genes needed for cell separation as cells enter G1. Early and late G1-specific transcription is mediated by the Swi5/Ace2 and Swi4/Swi6 classes of factor, respectively. Changes in cyclin/Cdc28 kinases may be involved in all classes of regulation. Transcriptional control of cyclin genes has an important role in regulating cell cycle progression.

pct1+, which encodes a new DNA-binding partner of p85cdc10, is required for meiosis in the fission yeast Schizosaccharomyces pombe

The transcriptional activation of genes at late G1 is an important regulatory step in the commitment to a new cell division cycle. In Schizosaccharomyces pombe, this regulation is mediated by MCB elements that serve as binding sites for the MBF/DSC-1 complex. The cdc10(+)-encoded protein is a component of this complex. We report the cloning of a new gene, pct1+, encoding a 73-kD protein that interacts with p85cdc10 to form an MCB-binding heteromer. Pct1+ is related to, but distinct from, the res1+/sct1+ gene that also encodes a p85cdc10 partner. p73pct1 has centrally located ankyrin repeats and a putative amino-terminal DNA-binding domain that has extensive sequence similarity to the DNA-binding domains of the Saccharomyces cerevisiae SWI4 and MBP1 proteins. The p73pct1/p85cdc10 complex binds both in vitro and in vivo to MCB but not SCB or E2F sites. Overexpression of pct1+ is sufficient to rescue the growth of the cdc10-129 temperature-sensitive mutant at the restrictive temperature, although it is unable to rescue a cdc10 null mutation. A deletion of pct1+ is not lethal but does result in a severe meiotic defect. Our results indicate that there are two cdc10-containing heteromeric complexes that bind to MCB elements and play differential roles in mitotic division and meiosis.