The yeast Sfp1 protein regulates both cell division and growth but how it coordinates these processes is poorly understood. We demonstrate that Sfp1 directly controls genes required for ribosome production and many other growth-promoting processes. Remarkably, the complete set of Sfp1 target genes is revealed only by a combination of ChIP (chromatin immunoprecipitation) and ChEC (chromatin endogenous cleavage) methods, which uncover two promoter binding modes, one requiring a cofactor and the other a DNA-recognition motif. Glucose-regulated Sfp1 binding at cell cycle "START" genes suggests that Sfp1 controls cell size by coordinating expression of genes implicated in mass accumulation and cell division.

While expression of ribosomal protein genes (RPGs) in the budding yeast has been extensively studied, a longstanding enigma persists regarding their co-regulation under fluctuating growth conditions. Most RPG promoters display one of two distinct arrangements of a core set of transcription factors (TFs) and are further differentiated by the presence or absence of the HMGB protein Hmo1. However, a third group of promoters appears not to be bound by any of these proteins, raising the question of how the whole suite of genes is co-regulated. We demonstrate here that all RPGs are regulated by two distinct, but complementary mechanisms driven by the TFs Ifh1 and Sfp1, both of which are required for maximal expression in optimal conditions and coordinated downregulation upon stress. At the majority of RPG promoters, Ifh1-dependent regulation predominates, whereas Sfp1 plays the major role at all other genes. We also uncovered an unexpected protein homeostasis-dependent binding property of Hmo1 at RPG promoters. Finally, we show that the Ifh1 paralog Crf1, previously described as a transcriptional repressor, can act as a constitutive RPG activator. Our study provides a more complete picture of RPG regulation and may serve as a paradigm for unravelling RPG regulation in multicellular eukaryotes.

Chromatin remodelling is essential for cardiac development. Interestingly, the role of histone chaperones has not been investigated in this regard. HIRA is a member of the HUCA (HIRA/UBN1/CABIN1/ASF1a) complex that deposits the variant histone H3.3 on chromatin independently of replication. Lack of HIRA has general effects on chromatin and gene expression dynamics in embryonic stem cells and mouse oocytes. Here we describe the conditional ablation of Hira in the cardiogenic mesoderm of mice. We observed surface oedema, ventricular and atrial septal defects and embryonic lethality. We identified dysregulation of a subset of cardiac genes, notably upregulation of troponins Tnni2 and Tnnt3, involved in cardiac contractility and decreased expression of Epha3, a gene necessary for the fusion of the muscular ventricular septum and the atrioventricular cushions. We found that HIRA binds GAGA rich DNA loci in the embryonic heart, and in particular a previously described enhancer of Tnni2/Tnnt3 (TTe) bound by the transcription factor NKX2.5. HIRA-dependent H3.3 enrichment was observed at the TTe in embryonic stem cells (ESC) differentiated toward cardiomyocytes in vitro. Thus, we show here that HIRA has locus-specific effects on gene expression and that histone chaperone activity is vital for normal heart development, impinging on pathways regulated by an established cardiac transcription factor.

Replication forks temporarily or terminally pause at hundreds of hard-to-replicate regions around the genome. A conserved pair of budding yeast replisome components Tof1-Csm3 (fission yeast Swi1-Swi3 and human TIMELESS -TIPIN) act as a "molecular brake" and promote fork slowdown at proteinaceous replication fork barriers (RFBs), while the accessory helicase Rrm3 assists the replisome in removing protein obstacles. Here we show that the Tof1-Csm3 complex promotes fork pausing independently of Rrm3 helicase by recruiting topoisomerase I (Top1) to the replisome. Topoisomerase II (Top2) partially compensates for the pausing decrease in cells when Top1 is lost from the replisome. The C terminus of Tof1 is specifically required for Top1 recruitment to the replisome and fork pausing but not for DNA replication checkpoint (DRC) activation. We propose that forks pause at proteinaceous RFBs through a "sTOP" mechanism ("slowing down with topoisomerases I-II"), which we show also contributes to protecting cells from topoisomerase-blocking agents.

16Understanding how transcriptional programs help to coordinate cell growth and division is an 17 important unresolved problem. Here we report that the nutrient-and stress-regulated transcription 18 factor Sfp1 is rate-limiting for expression of several large classes of genes involved in yeast cell growth, 19 including ribosomal protein, ribosome biogenesis, and snoRNA genes. Remarkably, the spectrum of 20 Sfp1 transcription effects is concordant with a combination of chromatin immunoprecipitation and 21 chromatin endogenous cleavage binding analyses, which together provide evidence for two distinct 22 modes of Sfp1 promoter binding, one requiring a co-factor and the other a specific DNA-recognition 23 motif. In addition to growth-related genes, Sfp1 binds to and regulates the promoters of cell cycle 24 "START" regulon genes, including the key G1/S cyclins CLN1 and CLN2. Our findings suggest that Sfp1 25 acts as a master regulator of cell growth and cell size by coordinating the expression of genes implicated 26 in mass accumulation and cell division. 2728 48 large-cell phenotype (Jorgensen et al., 2002). Taken together, these findings suggest that Sfp1 might 49 play a key role in coordinating cell growth and cell division. Interestingly, the transcriptional and cell-50 size phenotypes of SFP1 are notably similar to those of the c-Myc proto-oncogene (Jorgensen et al.,One paradox that has limited our understanding of Sfp1's mechanism of action is that the protein has 53 been detected by Chromatin Immuno-Precipitation (ChIP) at only a small fraction of the promoters that 54 it appears to regulate. For example, although ChIP detects Sfp1 at many RP gene promoters (Reja, 55 Vinayachandran et al., 2015), it is undetectable at virtually all of the >200 RiBi gene promoters where 56 4 over-expression studies suggest that it might be a direct activator (Jorgensen et al., 2002, Jorgensen et 57 al., 2004. 58Here we vastly expand our knowledge of Sfp1 binding by Chromatin Endogenous Cleavage (ChEC)-seq 59 analysis (Schmid, Durussel et al., 2004, Zentner, Kasinathan et al., 2015. Remarkably, we find that 60 ChEC and ChIP provide a highly complementary picture of Sfp1 binding, with distinct sets of sites 61 identified by one technique or the other. Our combined analysis provides evidence that Sfp1 directly 62 orchestrates TATA-binding protein (TBP) and RNAPII recruitment at a broad array of genes that drive 63 cell growth, including most RiBi, RP and snoRNA genes. In addition, we find that Sfp1 binds to the 64 promoters of many G1/S ("START") regulon genes that are targeted by the TF Swi4. Interestingly, Sfp1 65 binding sites identified by ChEC are enriched for the motif gAAAATTTTc, whereas binding identified by 66 ChIP is often strongly dependent on another TF: Ifh1 at RP genes or Swi4 at G1/S regulon genes. These 67 findings provide an unprecedented example of how the combination of ChIP and ChEC can reveal a 68 more complete picture of TF-chromatin interactions. Taken together, our results support a role for 69 Sfp1 as ...

The high-throughput 3D visualisation of biological specimens is essential for studying diseases and developmental disorders. It requires imaging methods that deliver high-contrast, high-resolution volumetric information at short sample preparation and acquisition times. Here we show that X-ray phase-contrast tomography using a single grating can provide a powerful alternative to commonly employed techniques, such as high-resolution episcopic microscopy (HREM). We present the phase tomography of a mouse embryo in paraffin obtained with an X-ray single-grating interferometer at I13-2 Beamline at Diamond Light Source and discuss the results in comparison with HREM measurements. The excellent contrast and quantitative density information achieved non-destructively and without staining using a simple, robust setup make X-ray single-grating interferometry an optimum candidate for high-throughput imaging of biological specimens as an alternative for existing methods like HREM.

Background The Ccr4-Not complex is mostly known as the major eukaryotic deadenylase. However, several studies have uncovered roles of the complex, in particular of the Not subunits, unrelated to deadenylation and relevant for translation. In particular, the existence of Not condensates that regulate translation elongation dynamics has been reported. Typical studies that evaluate translation efficiency rely on soluble extracts obtained after the disruption of cells and ribosome profiling. Yet cellular mRNAs in condensates can be actively translated and may not be present in such extracts. Results In this work, by analyzing soluble and insoluble mRNA decay intermediates in yeast, we determine that insoluble mRNAs are enriched for ribosomes dwelling at non-optimal codons compared to soluble mRNAs. mRNA decay is higher for soluble RNAs, but the proportion of co-translational degradation relative to the overall mRNA decay is higher for insoluble mRNAs. We show that depletion of Not1 and Not4 inversely impacts mRNA solubilities and, for soluble mRNAs, ribosome dwelling according to codon optimality. Depletion of Not4 solubilizes mRNAs with lower non-optimal codon content and higher expression that are rendered insoluble by Not1 depletion. By contrast, depletion of Not1 solubilizes mitochondrial mRNAs, which are rendered insoluble upon Not4 depletion. Conclusions Our results reveal that mRNA solubility defines the dynamics of co-translation events and is oppositely regulated by Not1 and Not4, a mechanism that we additionally determine may already be set by Not1 promoter association in the nucleus.

Background: The Ccr4-Not complex is most well known as the major eukaryotic deadenylase. However, several studies have uncovered roles of the complex, in particular of the Not subunits, unrelated to deadenylation and relevant for translation. In particular, the existence of Not condensates that regulate translation elongation dynamics have been reported. Typical studies that evaluate translation efficiency rely on soluble extracts obtained after disruption of cells and ribosome profiling. Yet cellular mRNAs in condensates can be actively translated and may not be present in such extracts. In this work, by analyzing soluble and insoluble mRNA decay intermediates in yeast, we determine that insoluble mRNAs are enriched for ribosomes dwelling at non-optimal codons compared to soluble mRNAs. mRNA decay is higher for soluble RNAs, but the proportion of co-translational degradation relative to the overall mRNA decay is higher for insoluble mRNAs. We show that depletion of Not1 and Not4 inversely impact mRNA solubilities and, for soluble mRNAs, ribosome dwelling according to codon optimality. Depletion of Not4 solubilizes mRNAs with lower non-optimal codon content and higher expression that are rendered insoluble by Not1 depletion. By contrast, depletion of Not1 solubilizes mitochondrial mRNAs, which are rendered insoluble upon Not4 depletion. Our results reveal that mRNA solubility defines dynamics of co-translation events and is oppositely regulated by Not1 and Not4, a mechanism that we additionally determine may already be set by Not1 promoter association in the nucleus.