Promoter architectures in the yeast ribosomal expression program

Compound K Production: Achievements and Perspectives

Compound K (CK) is one of the major metabolites found in mammalian blood and organs following oral administration of Panax plants. CK, also known as minor ginsenoside, can be absorbed in the systemic circulation. It has garnered significant attention in healthcare and medical products due to its pharmacological activities, such as antioxidation, anticancer, antiproliferation, antidiabetics, neuroprotection, and anti-atherogenic activities. However, CK is not found in natural ginseng plants but in traditional chemical synthesis, which uses toxic solvents and leads to environmental pollution during the harvest process. Moreover, enzymatic reactions are impractical for industrial CK production due to low yield and high costs. Although CK could be generated from major ginsenosides, most ginsenosides, including protopanaxatriol-oleanane and ocotillol-type, are not converted into CK by catalyzing β-glucosidase. Therefore, microbial cell systems have been used as a promising solution, providing a safe and efficient approach to CK production. This review provides a summary of various approaches for the production of CK, including chemical and enzymatic reactions, biotransformation by the human intestinal bacteria and endophytes as well as engineered microbes. Moreover, the approaches for CK production have been discussed to improve the productivity of target compounds.

Differential Hsp90-dependent gene expression is strain-specific and common among yeast strains

Enhanced phenotypic diversity increases a population's likelihood of surviving catastrophic conditions. Hsp90, an essential molecular chaperone and a central network hub in eukaryotes, has been observed to suppress or enhance the effects of genetic variation on phenotypic diversity in response to environmental cues. Because many Hsp90-interacting genes are involved in signaling transduction pathways and transcriptional regulation, we tested how common Hsp90-dependent differential gene expression is in natural populations. Many genes exhibited Hsp90-dependent strain-specific differential expression in five diverse yeast strains. We further identified transcription factors (TFs) potentially contributing to variable expression. We found that on Hsp90 inhibition or environmental stress, activities or abundances of Hsp90-dependent TFs varied among strains, resulting in differential strain-specific expression of their target genes, which consequently led to phenotypic diversity. We provide evidence that individual strains can readily display specific Hsp90-dependent gene expression, suggesting that the evolutionary impacts of Hsp90 are widespread in nature.

Interaction of TOR and PKA Signaling in S. cerevisiae

TOR and PKA signaling are the major growth-regulatory nutrient-sensing pathways in S. cerevisiae. A number of experimental findings demonstrated a close relationship between these pathways: Both are responsive to glucose availability. Both regulate ribosome production on the transcriptional level and repress autophagy and the cellular stress response. Sch9, a major downstream effector of TORC1 presumably shares its kinase consensus motif with PKA, and genetic rescue and synthetic defects between PKA and Sch9 have been known for a long time. Further, studies in the first decade of this century have suggested direct regulation of PKA by TORC1. Nonetheless, the contribution of a potential direct cross-talk vs. potential sharing of targets between the pathways has still not been completely resolved. What is more, other findings have in contrast highlighted an antagonistic relationship between the two pathways. In this review, I explore the association between TOR and PKA signaling, mainly by focusing on proteins that are commonly referred to as shared TOR and PKA targets. Most of these proteins are transcription factors which to a large part explain the major transcriptional responses elicited by TOR and PKA upon nutrient shifts. I examine the evidence that these proteins are indeed direct targets of both pathways and which aspects of their regulation are targeted by TOR and PKA. I further explore if they are phosphorylated on shared sites by PKA and Sch9 or when experimental findings point towards regulation via the PP2ASit4/PP2A branch downstream of TORC1. Finally, I critically review data suggesting direct cross-talk between the pathways and its potential mechanism.

Transcription at a Distance in the Budding Yeast Saccharomyces cerevisiae

Proper transcriptional regulation depends on the collaboration of multiple layers of control simultaneously. Cells tightly balance cellular resources and integrate various signaling inputs to maintain homeostasis during growth, development and stressors, among other signals. Many eukaryotes, including the budding yeast Saccharomyces cerevisiae, exhibit a non-random distribution of functionally related genes throughout their genomes. This arrangement coordinates the transcription of genes that are found in clusters, and can occur over long distances. In this work, we review the current literature pertaining to gene regulation at a distance in budding yeast.

Genomic clustering within functionally related gene families in Ascomycota fungi

Multiple mechanisms collaborate for proper regulation of gene expression. One layer of this regulation is through the clustering of functionally related genes at discrete loci throughout the genome. This phenomenon occurs extensively throughout Ascomycota fungi and is an organizing principle for many gene families whose proteins participate in diverse molecular functions throughout the cell. Members of this phylum include organisms that serve as model systems and those of interest medically, pharmaceutically, and for industrial and biotechnological applications. In this review, we discuss the prevalence of functional clustering through a broad range of organisms within the phylum. Position effects on transcription, genomic locations of clusters, transcriptional regulation of clusters, and selective pressures contributing to the formation and maintenance of clusters are addressed, as are common methods to identify and characterize clusters.

Regulation of ribosomal protein genes: An ordered anarchy

Ribosomal protein genes are among the most highly expressed genes in most cell types. Their products are generally essential for ribosome synthesis, which is the cornerstone for cell growth and proliferation. Many cellular resources are dedicated to producing ribosomal proteins and thus this process needs to be regulated in ways that carefully balance the supply of nascent ribosomal proteins with the demand for new ribosomes. Ribosomal protein genes have classically been viewed as a uniform interconnected regulon regulated in eukaryotic cells by target of rapamycin and protein kinase A pathway in response to changes in growth conditions and/or cellular status. However, recent literature depicts a more complex picture in which the amount of ribosomal proteins produced varies between genes in response to two overlapping regulatory circuits. The first includes the classical general ribosome‐producing program and the second is a gene‐specific feature responsible for fine‐tuning the amount of ribosomal proteins produced from each individual ribosomal gene. Unlike the general pathway that is mainly controlled at the level of transcription and translation, this specific regulation of ribosomal protein genes is largely achieved through changes in pre‐mRNA splicing efficiency and mRNA stability. By combining general and specific regulation, the cell can coordinate ribosome production, while allowing functional specialization and diversity. Here we review the many ways ribosomal protein genes are regulated, with special focus on the emerging role of posttranscriptional regulatory events in fine‐tuning the expression of ribosomal protein genes and its role in controlling the potential variation in ribosome functions.

This article is categorized under:

Translation > Ribosome Biogenesis

Translation > Ribosome Structure/Function

Translation > Translation Regulation

Genomic Considerations for the Modification of Saccharomyces cerevisiae for Biofuel and Metabolite Biosynthesis

The growing global population and developing world has put a strain on non-renewable natural resources, such as fuels. The shift to renewable sources will, thus, help meet demands, often through the modification of existing biosynthetic pathways or the introduction of novel pathways into non-native species. There are several useful biosynthetic pathways endogenous to organisms that are not conducive for the scale-up necessary for industrial use. The use of genetic and synthetic biological approaches to engineer these pathways in non-native organisms can help ameliorate these challenges. The budding yeast Saccharomyces cerevisiae offers several advantages for genetic engineering for this purpose due to its widespread use as a model system studied by many researchers. The focus of this review is to present a primer on understanding genomic considerations prior to genetic modification and manipulation of S. cerevisiae. The choice of a site for genetic manipulation can have broad implications on transcription throughout a region and this review will present the current understanding of position effects on transcription.

Pol5 is an essential ribosome biogenesis factor required for 60S ribosomal subunit maturation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, more than 250 trans-acting factors are involved in the maturation of 40S and 60S ribosomal subunits. The expression of most of these factors is transcriptionally coregulated to ensure correct ribosome production under a wide variety of environmental and intracellular conditions. Here, we identified the essential nucleolar Pol5 protein as a novel trans-acting factor required for the synthesis of 60S ribosomal subunits. Pol5 weakly and/or transiently associates with early to medium pre-60S ribosomal particles. Depletion of and temperature-sensitive mutations in Pol5 result in a deficiency of 60S ribosomal subunits and accumulation of half-mer polysomes. Both processing of 27SB pre-rRNA to mature 25S rRNA and release of pre-60S ribosomal particles from the nucle(ol)us to the cytoplasm are impaired in the Pol5-depleted strain. Moreover, we identified the genes encoding ribosomal proteins uL23 and eL27A as multicopy suppressors of the slow growth of a temperature-sensitive pol5 mutant. These results suggest that Pol5 could function in ensuring the correct folding of 25S rRNA domain III; thus, favoring the correct assembly of these two ribosomal proteins at their respective binding sites into medium pre-60S ribosomal particles. Pol5 is homologous to the human tumor suppressor Myb-binding protein 1A (MYBBP1A). However, expression of MYBBP1A failed to complement the lethal phenotype of a pol5 null mutant strain though interfered with 60S ribosomal subunit biogenesis.

Functional duplication of Rap1 in methylotrophic yeasts

The telomere regulator and transcription factor Rap1 is the only telomere protein conserved in yeasts and mammals. Its functional repertoire in budding yeasts is a particularly interesting field for investigation, given the high evolutionary diversity of this group of unicellular organisms. In the methylotrophic thermotolerant species Hansenula polymorpha DL-1 the RAP1 gene is duplicated (HpRAP1A and HpRAP1B). Here, we report the functional characterization of the two paralogues from H. polymorpha DL-1. We uncover distinct (but overlapping) DNA binding preferences of HpRap1A and HpRap1B proteins. We show that only HpRap1B is able to recognize telomeric DNA directly and to protect it from excessive recombination, whereas HpRap1A is associated with subtelomere regions. Furthermore, we identify specific binding sites for both HpRap1A and HpRap1B within promoters of a large number of ribosomal protein genes (RPGs), implicating Rap1 in the control of the RP regulon in H. polymorpha. Our bioinformatic analysis suggests that RAP1 was duplicated early in the evolution of the “methylotrophs” clade, and the two genes evolved independently. Therefore, our characterization of Rap1 paralogues in H. polymorpha may be relevant to other “methylotrophs”, yielding valuable insights into the evolution of budding yeasts.

Functionally Related Genes Cluster into Genomic Regions That Coordinate Transcription at a Distance in Saccharomyces cerevisiae

The two-dimensional, physical positioning of genes along a chromosome can impact proper transcriptional regulation throughout a genomic region. The transcription of neighboring genes is correlated in a genome-wide manner, which is a characteristic of eukaryotes. Many coregulated gene families can be found clustered with another member of the same set—which can result in adjacent gene coregulation of the pair. Due to the myriad gene families that exhibit a nonrandom genomic distribution, there are likely multiple mechanisms working in concert to properly regulate transcriptional coordination of functionally clustered genes. In this study, we utilized budding yeast in an attempt to elucidate mechanisms that underlie this coregulation: testing and empirically validating the enhancer-promoter hypothesis in this species and reporting that functionally related genes cluster to genomic regions that are more conducive to transcriptional regulation at a distance. These clusters rely, in part, on chromatin maintenance and remodelers to maintain proper transcriptional coordination. Our work provides insight into the mechanisms underlying adjacent gene coregulation.

Balancing gene expression is a fundamental challenge of all cell types. To properly regulate transcription on a genome-wide level, there are myriad mechanisms employed by the cell. One layer to this regulation is through spatial positioning, with particular chromosomal loci exerting an influence on transcription throughout a region. Many coregulated gene families utilize spatial positioning to coordinate transcription, with functionally related genes clustering together which can allow coordinated expression via adjacent gene coregulation. The mechanisms underlying this process have not been elucidated, though there are many coregulated gene families that exhibit this genomic distribution. In the present study, we tested for a role for the enhancer-promoter (EP) hypothesis, which demonstrates that regulatory elements can exert transcriptional effects over a broad distance, in coordinating transcriptional coregulation using budding yeast, Saccharomyces cerevisiae. We empirically validated the EP model, finding that the genomic distance a promoter can affect varies by locus, which can profoundly affect levels of transcription, phenotype, and the extent of transcriptional disruption throughout a genomic region. Using the nitrogen metabolism, ribosomal protein, toxin response, and heat shock gene families as our test case, we report functionally clustered genes localize to genomic loci that are more conducive to transcriptional regulation at a distance compared to the unpaired members of the same families. Furthermore, we report that the coregulation of functional clusters is dependent, in part, on chromatin maintenance and remodeling, providing one mechanism underlying adjacent gene coregulation.

IMPORTANCE The two-dimensional, physical positioning of genes along a chromosome can impact proper transcriptional regulation throughout a genomic region. The transcription of neighboring genes is correlated in a genome-wide manner, which is a characteristic of eukaryotes. Many coregulated gene families can be found clustered with another member of the same set—which can result in adjacent gene coregulation of the pair. Due to the myriad gene families that exhibit a nonrandom genomic distribution, there are likely multiple mechanisms working in concert to properly regulate transcriptional coordination of functionally clustered genes. In this study, we utilized budding yeast in an attempt to elucidate mechanisms that underlie this coregulation: testing and empirically validating the enhancer-promoter hypothesis in this species and reporting that functionally related genes cluster to genomic regions that are more conducive to transcriptional regulation at a distance. These clusters rely, in part, on chromatin maintenance and remodelers to maintain proper transcriptional coordination. Our work provides insight into the mechanisms underlying adjacent gene coregulation.

Sfp1 regulates transcriptional networks driving cell growth and division through multiple promoter-binding modes

In this study, Albert et al. investigated the mechanisms by which the yeast Sfp1 protein coordinates both cell division and growth. They demonstrate that Sfp1 directly controls genes required for ribosome production and many other growth-promoting processes.

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.

Systematic Identification, Characterization, and Conservation of Adjacent-Gene Coregulation in the Budding Yeast Saccharomyces cerevisiae

The spatial positioning of genes throughout the genome arrangement can alter their expression in many eukaryotic organisms. Often this results in a genomic context-specific effect on transcription. One example of this is through the clustering of functionally related genes, which results in adjacent-gene coregulation in the budding yeast Saccharomyces cerevisiae. In the present study, we set out to systematically characterize the prevalence of this phenomenon, finding the genomic organization of functionally related genes into clusters is a characteristic of myriad gene families. These arrangements are found in many evolutionarily divergent fungi and thus represent a widespread, yet underappreciated, layer of transcriptional regulation.

It is essential that cells orchestrate gene expression for the specific niche that they occupy, and this often requires coordination of the expression of large sets of genes. There are multiple regulatory systems that exist for modulation of gene expression, including the adjacent-gene coregulation of the rRNA and ribosome biogenesis and ribosomal protein families. Both gene families exhibit a nonrandom genomic distribution, often clustered directly adjacent to another member of the same family, which results in a tighter transcriptional coordination among adjacent paired genes than that of the unpaired genes within each regulon and can result in a shared promoter that coordinates expression of the pairs. This nonrandom genomic distribution has been seen in a few functionally related gene families, and many of these functional pairings are conserved across divergent fungal lineages. To date, the significance of these observations has not been extended in a systematic way to characterize how prevalent the role of adjacent-gene coregulation is in transcriptional regulation. In the present study, we systematically analyzed the transcriptional coherence of the functional pairs compared to the singletons within all gene families defined by the Gene Ontology Slim designation, using Saccharomyces cerevisiae as a model system, finding that clusters exhibit a tighter transcriptional correlation under specific contexts. We found that the longer a functional pairing is conserved the tighter its response to broad stress and nutritional responses, that roughly 25% of gene families exhibit a nonrandom genomic distribution, and that many of these clusters are conserved. This suggests that adjacent-gene coregulation is a widespread, yet underappreciated, transcriptional mechanism.

IMPORTANCE The spatial positioning of genes throughout the genome arrangement can alter their expression in many eukaryotic organisms. Often this results in a genomic context-specific effect on transcription. One example of this is through the clustering of functionally related genes, which results in adjacent-gene coregulation in the budding yeast Saccharomyces cerevisiae. In the present study, we set out to systematically characterize the prevalence of this phenomenon, finding the genomic organization of functionally related genes into clusters is a characteristic of myriad gene families. These arrangements are found in many evolutionarily divergent fungi and thus represent a widespread, yet underappreciated, layer of transcriptional regulation.

The ribosome assembly gene network is controlled by the feedback regulation of transcription elongation

Ribosome assembly requires the concerted expression of hundreds of genes, which are transcribed by all three nuclear RNA polymerases. Transcription elongation involves dynamic interactions between RNA polymerases and chromatin. We performed a synthetic lethal screening in Saccharomyces cerevisiae with a conditional allele of SPT6, which encodes one of the factors that facilitates this process. Some of these synthetic mutants corresponded to factors that facilitate pre-rRNA processing and ribosome biogenesis. We found that the in vivo depletion of one of these factors, Arb1, activated transcription elongation in the set of genes involved directly in ribosome assembly. Under these depletion conditions, Spt6 was physically targeted to the up-regulated genes, where it helped maintain their chromatin integrity and the synthesis of properly stable mRNAs. The mRNA profiles of a large set of ribosome biogenesis mutants confirmed the existence of a feedback regulatory network among ribosome assembly genes. The transcriptional response in this network depended on both the specific malfunction and the role of the regulated gene. In accordance with our screening, Spt6 positively contributed to the optimal operation of this global network. On the whole, this work uncovers a feedback control of ribosome biogenesis by fine-tuning transcription elongation in ribosome assembly factor-coding genes.

Feedback regulation of ribosome assembly

Ribosome biogenesis is a crucial process for growth and constitutes the major consumer of cellular resources. This pathway is subjected to very stringent regulation to ensure correct ribosome manufacture with a wide variety of environmental and metabolic changes, and intracellular insults. Here we summarise our current knowledge on the regulation of ribosome biogenesis in Saccharomyces cerevisiae by particularly focusing on the feedback mechanisms that maintain ribosome homeostasis. Ribosome biogenesis in yeast is controlled mainly at the level of the production of both pre-rRNAs and ribosomal proteins through the transcriptional and post-transcriptional control of the TORC1 and protein kinase A signalling pathways. Pre-rRNA processing can occur before or after the 35S pre-rRNA transcript is completed; the switch between these two alternatives is regulated by growth conditions. The expression of both ribosomal proteins and the large family of transacting factors involved in ribosome biogenesis is co-regulated. Recently, it has been shown that the synthesis of rRNA and ribosomal proteins, but not of trans-factors, is coupled. Thus the so-called CURI complex sequesters specific transcription factor Ifh1 to repress ribosomal protein genes when rRNA transcription is impaired. We recently found that an analogue system should operate to control the expression of transacting factor genes in response to actual ribosome assembly performance. Regulation of ribosome biogenesis manages situations of imbalanced ribosome production or misassembled ribosomal precursors and subunits, which have been closely linked to distinct human diseases.

Abf1 and other general regulatory factors control ribosome biogenesis gene expression in budding yeast

Ribosome biogenesis in Saccharomyces cerevisiae involves a regulon of >200 genes (Ribi genes) coordinately regulated in response to nutrient availability and cellular growth rate. Two cis-acting elements called PAC and RRPE are known to mediate Ribi gene repression in response to nutritional downshift. Here, we show that most Ribi gene promoters also contain binding sites for one or more General Regulatory Factors (GRFs), most frequently Abf1 and Reb1, and that these factors are enriched in vivo at Ribi promoters. Abf1/Reb1/Tbf1 promoter association was required for full Ribi gene expression in rich medium and for its modulation in response to glucose starvation, characterized by a rapid drop followed by slow recovery. Such a response did not entail changes in Abf1 occupancy, but it was paralleled by a quick increase, followed by slow decrease, in Rpd3L histone deacetylase occupancy. Remarkably, Abf1 site disruption also abolished Rpd3L complex recruitment in response to starvation. Extensive mutational analysis of the DBP7 promoter revealed a complex interplay of Tbf1 sites, PAC and RRPE in the transcriptional regulation of this Ribi gene. Our observations point to GRFs as new multifaceted players in Ribi gene regulation both during exponential growth and under repressive conditions.

Transcriptional control of yeast ribosome biogenesis: A multifaceted role for general regulatory factors

In Saccharomyces cerevisiae, a group of more than 200 co-regulated genes (Ribi genes) is involved in ribosome biogenesis. This regulon has recently been shown to rely on a small set of transcriptional regulators (mainly Abf1, but also Reb1, Tbf1 and Rap1) previously referred to as general regulatory factors (GRFs) because of their widespread binding and action at many promoters and other specialized genomic regions. Intriguingly, Abf1 binding to Ribi genes is differentially modulated in response to distinct nutrition signaling pathways. Such a dynamic promoter association has the potential to orchestrate both activation and repression of Ribi genes in synergy with neighboring regulatory sites and through the functional interplay of histone acetyltransferases and deacetylases.

Growth rate controls mRNA turnover in steady and non-steady states

Gene expression has been investigated in relation with growth rate in the yeast Saccharomyces cerevisiae, following different experimental strategies. The expression of some specific gene functional categories increases or decreases with growth rate. Our recently published results have unveiled that these changes in mRNA concentration with growth depend on the relative alteration of mRNA synthesis and decay, and that, in addition to this gene-specific transcriptomic signature of growth, global mRNA turnover increases with growth rate. We discuss here these results in relation with other previous and concurrent publications, and we add new evidence which indicates that growth rate controls mRNA turnover even under non-steady-state conditions.

Promoter architecture and transcriptional regulation of Abf1-dependent ribosomal protein genes in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, ribosomal protein gene (RPG) promoters display binding sites for either Rap1 or Abf1 transcription factors. Unlike Rap1-associated promoters, the small cohort of Abf1-dependent RPGs (Abf1-RPGs) has not been extensively investigated. We show that RPL3, RPL4B, RPP1A, RPS22B and RPS28A/B share a common promoter architecture, with an Abf1 site upstream of a conserved element matching the sequence recognized by Fhl1, a transcription factor which together with Ifh1 orchestrates Rap1-associated RPG regulation. Abf1 and Fhl1 promoter association was confirmed by ChIP and/or gel retardation assays. Mutational analysis revealed a more severe requirement of Abf1 than Fhl1 binding sites for RPG transcription. In the case of RPS22B an unusual Tbf1 binding site promoted both RPS22B and intron-hosted SNR44 expression. Abf1-RPG down-regulation upon TOR pathway inhibition was much attenuated at defective mutant promoters unable to bind Abf1. TORC1 inactivation caused the expected reduction of Ifh1 occupancy at RPS22B and RPL3 promoters, but unexpectedly it entailed largely increased Abf1 association with Abf1-RPG promoters. We present evidence that Abf1 recruitment upon nutritional stress, also observed for representative ribosome biogenesis genes, favours RPG transcriptional rescue upon nutrient replenishment, thus pointing to nutrient-regulated Abf1 dynamics at promoters as a novel mechanism in ribosome biogenesis control.

Introns regulate the production of ribosomal proteins by modulating splicing of duplicated ribosomal protein genes

Most budding yeast introns exist in the many duplicated ribosomal protein genes (RPGs) and it has been posited that they remain there to modulate the expression of RPGs and cell growth in response to stress. However, the mechanism by which introns regulate the expression of RPGs and their impact on the synthesis of ribosomal proteins remain unclear. In this study, we show that introns determine the ratio of ribosomal protein isoforms through asymmetric paralog-specific regulation of splicing. Exchanging the introns and 3′ untranslated regions of the duplicated RPS9 genes altered the splicing efficiency and changed the ratio of the ribosomal protein isoforms. Mutational analysis of the RPS9 genes indicated that splicing is regulated by variations in the intron structure and the 3′ untranslated region. Together these data suggest that preferential splicing of duplicated RPGs provides a means for adjusting the ratio of different ribosomal protein isoforms, while maintaining the overall expression level of each ribosomal protein.

Prediction of fine-tuned promoter activity from DNA sequence

The quantitative prediction of transcriptional activity of genes using promoter sequence is fundamental to the engineering of biological systems for industrial purposes and understanding the natural variation in gene expression. To catalyze the development of new algorithms for this purpose, the Dialogue on Reverse Engineering Assessment and Methods (DREAM) organized a community challenge seeking predictive models of promoter activity given normalized promoter activity data for 90 ribosomal protein promoters driving expression of a fluorescent reporter gene. By developing an unbiased modeling approach that performs an iterative search for predictive DNA sequence features using the frequencies of various k-mers, inferred DNA mechanical properties and spatial positions of promoter sequences, we achieved the best performer status in this challenge. The specific predictive features used in the model included the frequency of the nucleotide G, the length of polymeric tracts of T and TA, the frequencies of 6 distinct trinucleotides and 12 tetranucleotides, and the predicted protein deformability of the DNA sequence. Our method accurately predicted the activity of 20 natural variants of ribosomal protein promoters (Spearman correlation r = 0.73) as compared to 33 laboratory-mutated variants of the promoters (r = 0.57) in a test set that was hidden from participants. Notably, our model differed substantially from the rest in 2 main ways: i) it did not explicitly utilize transcription factor binding information implying that subtle DNA sequence features are highly associated with gene expression, and ii) it was entirely based on features extracted exclusively from the 100 bp region upstream from the translational start site demonstrating that this region encodes much of the overall promoter activity. The findings from this study have important implications for the engineering of predictable gene expression systems and the evolution of gene expression in naturally occurring biological systems.

Molecular mechanisms of ribosomal protein gene coregulation

The 137 ribosomal protein genes (RPG) of Saccharomyces provide a model for gene coregulation. Reja et al. examine the positional and functional organization of their regulators (Rap1, Fhl1, Ifh1, Sfp1, and Hmo1), the transcription machinery (TFIIB, TFIID, and RNA polymerase II), and chromatin at near-base-pair resolution using ChIP-exo.

The 137 ribosomal protein genes (RPGs) of Saccharomyces provide a model for gene coregulation. We examined the positional and functional organization of their regulators (Rap1 [repressor activator protein 1], Fhl1, Ifh1, Sfp1, and Hmo1), the transcription machinery (TFIIB, TFIID, and RNA polymerase II), and chromatin at near-base-pair resolution using ChIP-exo, as RPGs are coordinately reprogrammed. Where Hmo1 is enriched, Fhl1, Ifh1, Sfp1, and Hmo1 cross-linked broadly to promoter DNA in an RPG-specific manner and demarcated by general minor groove widening. Importantly, Hmo1 extended 20–50 base pairs (bp) downstream from Fhl1. Upon RPG repression, Fhl1 remained in place. Hmo1 dissociated, which was coupled to an upstream shift of the +1 nucleosome, as reflected by the Hmo1 extension and core promoter region. Fhl1 and Hmo1 may create two regulatable and positionally distinct barriers, against which chromatin remodelers position the +1 nucleosome into either an activating or a repressive state. Consistent with in vitro studies, we found that specific TFIID subunits, in addition to cross-linking at the core promoter, made precise cross-links at Rap1 sites, which we interpret to reflect native Rap1–TFIID interactions. Our findings suggest how sequence-specific DNA binding regulates nucleosome positioning and transcription complex assembly >300 bp away and how coregulation coevolved with coding sequences.

TRF2, but not TBP, mediates the transcription of ribosomal protein genes

The TCT core promoter element is present in most ribosomal protein (RP) genes in Drosophila and humans. Here we show that TBP (TATA box-binding protein)-related factor TRF2, but not TBP, is required for transcription of the TCT-dependent RP genes. In cells, TCT-dependent transcription, but not TATA-dependent transcription, increases or decreases upon overexpression or depletion of TRF2. In vitro, purified TRF2 activates TCT but not TATA promoters. ChIP-seq (chromatin immunoprecipitation [ChIP] combined with deep sequencing) experiments revealed the preferential localization of TRF2 at TCT versus TATA promoters. Hence, a specialized TRF2-based RNA polymerase II system functions in the synthesis of RPs and complements the RNA polymerase I and III systems.

Dissecting the cis and trans elements that regulate adjacent-gene coregulation in Saccharomyces cerevisiae

The relative positions that genes occupy on their respective chromosomes can play a critical role in determining how they are regulated at the transcriptional level. For example, a significant fraction of the genes from a variety of coregulated gene sets, including the ribosomal protein (RP) and the rRNA and ribosome biogenesis (RRB) regulons, exist as immediate, adjacent gene pairs. These gene pairs occur in all possible divergent, tandem, and convergent orientations. Adjacent-gene pairing in these regulons is associated with a tighter transcriptional coregulation than is observed for nonpaired genes of the same regulons. In order to define the cis and trans factors that regulate adjacent-gene coregulation (AGC), we conducted a mutational analysis of the convergently oriented RRB gene pair MPP10-YJR003C. We observed that coupled corepression of the gene pair under heat shock was abrogated when the two genes were separated by an actively expressed RNA polymerase (Pol) II transcription unit (the LEU2 gene) but not when the inserted LEU2 gene was repressed. In contrast, the insertion of an RNA Pol III-transcribed tRNA (Thr) gene did not disrupt the coregulated repression of MPP10 and YJR003C. A targeted screen of mutants defective in regulating chromosome architecture revealed that the Spt20, Snf2, and Chd1 proteins were required for coupling the repression of YJR003C to that of MPP10. Nucleosome occupancy assays performed across the MPP10 and YJR003C promoter regions revealed that the mechanism of corepression of the gene pair was not related to the repositioning of nucleosomes across the respective gene promoters.

A role for the nucleoporin Nup170p in chromatin structure and gene silencing

Embedded in the nuclear envelope, nuclear pore complexes (NPCs) not only regulate nuclear transport but also interface with transcriptionally active euchromatin, largely silenced heterochromatin, as well as the boundaries between these regions. It is unclear what functional role NPCs play in establishing or maintaining these distinct chromatin domains. We report that the yeast NPC protein Nup170p interacts with regions of the genome that contain ribosomal protein and subtelomeric genes, where it functions in nucleosome positioning and as a repressor of transcription. We show that the role of Nup170p in subtelomeric gene silencing is linked to its association with the RSC chromatin-remodeling complex and the silencing factor Sir4p, and that the binding of Nup170p and Sir4p to subtelomeric chromatin is cooperative and necessary for the association of telomeres with the nuclear envelope. Our results establish the NPC as an active participant in silencing and the formation of peripheral heterochromatin.

Driving the cell cycle through metabolism

For unicellular organisms, the decision to enter the cell cycle can be viewed most fundamentally as a metabolic problem. A cell must assess its nutritional and metabolic status to ensure it can synthesize sufficient biomass to produce a new daughter cell. The cell must then direct the appropriate metabolic outputs to ensure completion of the division process. Herein, we discuss the changes in metabolism that accompany entry to, and exit from, the cell cycle for the unicellular eukaryote Saccharomyces cerevisiae. Studies of budding yeast under continuous, slow-growth conditions have provided insights into the essence of these metabolic changes at unprecedented temporal resolution. Some of these mechanisms by which cell growth and proliferation are coordinated with metabolism are likely to be conserved in multicellular organisms. An improved understanding of the metabolic basis of cell cycle control promises to reveal fundamental principles governing tumorigenesis, metazoan development, niche expansion, and many additional aspects of cell and organismal growth control.

Genomic insights into the different layers of gene regulation in yeast

The model organism Saccharomyces cerevisiae has allowed the development of new functional genomics techniques devoted to the study of transcription in all its stages. With these techniques, it has been possible to find interesting new mechanisms to control gene expression that act at different levels and for different gene sets apart from the known cis-trans regulation in the transcription initiation step. Here we discuss a method developed in our laboratory, Genomic Run-On, and other new methods that have recently appeared with distinct technical features. A comparison between the datasets generated by them provides interesting genomic insights into the different layers of gene regulation in eukaryotes.

RNA polymerase I activity is regulated at multiple steps in the transcription cycle: recent insights into factors that influence transcription elongation

Synthesis of the translation apparatus is a central activity in growing and/or proliferating cells. Because of its fundamental importance and direct connection to cell proliferation, ribosome synthesis has been a focus of ongoing research for several decades. As a consequence, much is known about the essential factors involved in this process. Many studies have shown that transcription of the ribosomal DNA by RNA polymerase I is a major target for cellular regulation of ribosome synthesis rates. The initiation of transcription by RNA polymerase I has been implicated as a regulatory target, however, recent studies suggest that the elongation step in transcription is also influenced and regulated by trans-acting factors. This review describes the factors required for rRNA synthesis and focuses on recent works that have begun to identify and characterize factors that influence transcription elongation by RNA polymerase I and its regulation.