Nucleosome eviction in mitosis assists condensin loading and chromosome condensation.

Condensins associate with DNA and shape mitotic chromosomes. Condensins are enriched nearby highly expressed genes during mitosis, but how this binding is achieved and what features associated with transcription attract condensins remain unclear. Here, we report that condensin accumulates at or in the immediate vicinity of nucleosome-depleted regions during fission yeast mitosis. Two transcriptional coactivators, the Gcn5 histone acetyltransferase and the RSC chromatin-remodelling complex, bind to promoters adjoining condensin-binding sites and locally evict nucleosomes to facilitate condensin binding and allow efficient mitotic chromosome condensation. The function of Gcn5 is closely linked to condensin positioning, since neither the localization of topoisomerase II nor that of the cohesin loader Mis4 is altered in gcn5 mutant cells. We propose that nucleosomes act as a barrier for the initial binding of condensin and that nucleosome-depleted regions formed at highly expressed genes by transcriptional coactivators constitute access points into chromosomes where condensin binds free genomic DNA.

A genetic screen for functional partners of condensin in fission yeast.

Mitotic chromosome condensation is a prerequisite for the accurate segregation of chromosomes during cell division, and the conserved condensin complex a central player of this process. However, how condensin binds chromatin and shapes mitotic chromosomes remain poorly understood. Recent genome-wide binding studies showing that in most species condensin is enriched near highly expressed genes suggest a conserved link between condensin occupancy and high transcription rates. To gain insight into the mechanisms of condensin binding and mitotic chromosome condensation, we searched for factors that collaborate with condensin through a synthetic lethal genetic screen in the fission yeast Schizosaccharomyces pombe. We isolated novel mutations affecting condensin, as well as mutations in four genes not previously implicated in mitotic chromosome condensation in fission yeast. These mutations cause chromosome segregation defects similar to those provoked by defects in condensation. We also identified a suppressor of the cut3-477 condensin mutation, which largely rescued chromosome segregation during anaphase. Remarkably, of the five genes identified in this study, four encode transcription co-factors. Our results therefore provide strong additional evidence for a functional connection between chromosome condensation and transcription.

Natural sequence variants of yeast environmental sensors confer cell-to-cell expression variability.

Living systems may have evolved probabilistic bet hedging strategies that generate cell-to-cell phenotypic diversity in anticipation of environmental catastrophes, as opposed to adaptation via a deterministic response to environmental changes. Evolution of bet hedging assumes that genotypes segregating in natural populations modulate the level of intraclonal diversity, which so far has largely remained hypothetical. Using a fluorescent P(met17)-GFP reporter, we mapped four genetic loci conferring to a wild yeast strain an elevated cell-to-cell variability in the expression of MET17, a gene regulated by the methionine pathway. A frameshift mutation in the Erc1p transmembrane transporter, probably resulting from a release of laboratory strains from negative selection, reduced P(met17)-GFP expression variability. At a second locus, cis-regulatory polymorphisms increased mean expression of the Mup1p methionine permease, causing increased expression variability in trans. These results demonstrate that an expression quantitative trait locus (eQTL) can simultaneously have a deterministic effect in cis and a probabilistic effect in trans. Our observations indicate that the evolution of transmembrane transporter genes can tune intraclonal variation and may therefore be implicated in both reactive and anticipatory strategies of adaptation.

Fatty acid desaturation links germ cell loss to longevity through NHR-80/HNF4 in C. elegans.

BACKGROUND: Preventing germline stem cell proliferation extends lifespan in nematodes and flies. So far, studies on germline-longevity signaling have focused on daf-16/FOXO and daf-12/VDR. Here, we report on NHR-80/HNF4, a nuclear receptor that specifically mediates longevity induced by depletion of the germ line through a mechanism that implicates fatty acid monodesaturation. METHODS AND FINDINGS: nhr-80/HNF4 is induced in animals lacking a germ line and is specifically required for their extended longevity. Overexpressing nhr-80/HNF4 increases the lifespan of germline-less animals. This lifespan extension can occur in the absence of daf-16/FOXO but requires the presence of the nuclear receptor DAF-12/VDR. We show that the fatty acid desaturase, FAT-6/SCD1, is a key target of NHR-80/HNF4 and promotes germline-longevity by desaturating stearic acid to oleic acid (OA). We find that NHR-80/HNF4 and OA must work in concert to promote longevity. CONCLUSIONS: Taken together, our data indicate that the NHR-80 pathway participates in the mechanism of longevity extension through depletion of the germ line. We identify fat-6 and OA as essential downstream elements although other targets must also be present. Thus, NHR-80 links fatty acid desaturation to lifespan extension through germline ablation in a daf-16/FOXO independent manner.

Alternative usage of 5' exons in the chicken nerve growth factor gene: refined characterization of a weakly expressed gene.

Nerve growth factor (NGF) is the prototype member of the neurotrophin family. Identification of transcript structures and promoter regions is described here in view of clarifying the molecular basis of chicken NGF gene regulation. Chicken NGF complementary DNA (cDNA) was amplified from heart and brain mRNA using the single-strand ligation to cDNA (SLIC) procedure. Several cloning and sequencing rounds were necessary to elucidate the diversity of NGF transcripts. The chicken NGF gene was shown to possess, in addition to its unique 3' coding exon, five 5' exons grouped into two clusters that have been entirely sequenced. The first cluster encompasses three leader exons (1a, 1b and 1c) and is separated from the second cluster by a approximately 15 kilobases (kb) intronic sequence. "Exon walking" based on reverse transcription-polymerase chain reaction (RT-PCR) allowed to ascertain the length of the three leader exons. The second cluster contains exons 2 and 3, separated from each other by a approximately 2.4 kb intron, and lies approximately 0.5 kb upstream from coding exon 4. Combination of several mechanisms, such as differential usage of leader and internal exons, alternative transcription start inside exon 1b, second donor and acceptor sites in exon 1c and 4, respectively, leads to the production of at least 21 different transcripts. This remarkable diversity may represent a common feature largely underestimated for other weakly expressed genes. Preliminary RT-PCR expression study in a panel of chicken tissues shows that transcripts containing exon 1b are present in most tissues tested. Transcripts containing exon 1a are represented mainly in heart and reproductive organs, whereas transcripts containing exon 1c are mostly represented in peripheral organs other than heart. Complementary data are published as a Web supplement available at.