Cells use strategic metabolites to sense the metabolome and accordingly modulate gene expression. Here, we show that the purine and phosphate pathways are positively regulated by the metabolic intermediate AICAR (59-phosphoribosyl-5-amino-4-imidazole carboxamide). The transcription factor Pho2p is required for up-regulation of all AICAR-responsive genes. Accordingly, the binding of Pho2p to purine and phosphate pathway gene promoters is enhanced upon AICAR accumulation. In vitro, AICAR binds both Pho2p and Pho4p transcription factors and stimulates the interaction between Pho2p and either Bas1p or Pho4p in vivo. In contrast, SAICAR (succinyl-AICAR) only affects Pho2p-Bas1p interaction and specifically up-regulates purine regulon genes. Together, our data show that Bas1p and Pho4p compete for Pho2p binding, hence leading to the concerted regulation of cellular nucleotide synthesis and phosphate consumption.[Keywords: Phosphate utilization; purine pathway; small molecules; transcription factor; yeast] Supplemental material is available at http://www.genesdev.org.

In budding yeast and humans, cohesion establishment during S phase requires the acetyltransferase Eco1/Esco1-2, which acetylates the cohesin subunit Smc3 on two conserved lysine residues. Whether Smc3 is the sole Eco1/Esco1-2 effector and how Smc3 acetylation promotes cohesion are unknown. In fission yeast (Schizosaccharomyces pombe), as in humans, cohesin binding to G 1 chromosomes is dynamic and the unloading reaction is stimulated by Wpl1 (human ortholog, Wapl). During S phase, a subpopulation of cohesin becomes stably bound to chromatin in an Eso1 (fission yeast Eco1/Esco1-2)-dependent manner. Cohesin stabilization occurs unevenly along chromosomes. Cohesin remains largely labile at the rDNA repeats but binds mostly in the stable mode to pericentromere regions. This pattern is largely unchanged in eso1⌬ wpl1⌬ cells, and cohesion is unaffected, indicating that the main Eso1 role is counteracting Wpl1. A mutant of Psm3 (fission yeast Smc3) that mimics its acetylated state renders cohesin less sensitive to Wpl1-dependent unloading and partially bypasses the Eso1 requirement but cannot generate the stable mode of cohesin binding in the absence of Eso1. Conversely, nonacetylatable Psm3 reduces the stable cohesin fraction and affects cohesion in a Wpl1-dependent manner, but cells are viable. We propose that Psm3 acetylation contributes to Eso1 counteracting of Wpl1 to secure stable cohesin interaction with postreplicative chromosomes but that it is not the sole molecular event by which this occurs.Following DNA replication in S phase, sister DNA molecules are linked together by cohesin. Thereafter, cohesion between sister chromatids persists throughout the G 2 phase and until mitosis, where it allows chromosome biorientation on the mitotic spindle (16). Defects in this process have been linked to aneuploidy and tumor progression. Cohesin is a multisubunit protein complex made of a dimer of long, flexible Smc subunits, which form a ring-shaped structure stabilized by the binding of a kleisin subunit (Scc1/Rad21 in the mitotic cycle; Rec8 in meiosis) (2, 25). Kleisin cleavage by separase destroys the ring and allows chromatid separation at anaphase (28,43,58). The ring shape of the complex suggested that cohesin ensures cohesion by topological trapping of sister DNA molecules, and strong experimental evidence supports this model (24), although other modes of cohesin-DNA interaction might coexist (29,40).One key aspect of the cohesion cycle is how cohesion is made during S phase. Cohesin is first deposited on unreplicated chromosomes in a reaction requiring ATP hydrolysis by the Smc heads and the cohesin-loading complex Scc2/Scc4 (4,5,15,61). In an unperturbed cell cycle, cohesion is made exclusively during S phase, and except in the event of a DNA double-strand break (DSB), cohesin loading after DNA replication does not result in functional cohesion (26,34,51,60). Numerous studies have shown that mutations in nonessential factors associated with the replication fork machinery affect sister chromatid cohesion, leadin...

Sister chromatid cohesion is mediated by cohesin, but the process of cohesion establishment during S-phase is still enigmatic. In mammalian cells, cohesin binding to chromatin is dynamic in G1, but becomes stabilized during S-phase. Whether the regulation of cohesin stability is integral to the process of cohesion establishment is unknown. Here, we provide evidence that fission yeast cohesin also displays dynamic behavior. Cohesin association with G1 chromosomes requires continued activity of the cohesin loader Mis4/Ssl3, suggesting that repeated loading cycles maintain cohesin binding. Cohesin instability in G1 depends on wpl1, the fission yeast ortholog of mammalian Wapl, suggestive of a conserved mechanism that controls cohesin stability on chromosomes. wpl1 is nonessential, indicating that a change in wpl1-dependent cohesin dynamics is dispensable for cohesion establishment. Instead, we find that cohesin stability increases at the time of S-phase in a reaction that can be uncoupled from DNA replication. Hence, cohesin stabilization might be a pre-requisite for cohesion establishment rather than its consequence.