Two distinct promoter architectures centered on dynamic nucleosomes control ribosomal protein gene transcription.

In yeast, ribosome production is controlled transcriptionally by tight coregulation of the 138 ribosomal protein genes (RPGs). RPG promoters display limited sequence homology, and the molecular basis for their coregulation remains largely unknown. Here we identify two prevalent RPG promoter types, both characterized by upstream binding of the general transcription factor (TF) Rap1 followed by the RPG-specific Fhl1/Ifh1 pair, with one type also binding the HMG-B protein Hmo1. We show that the regulatory properties of the two promoter types are remarkably similar, suggesting that they are determined to a large extent by Rap1 and the Fhl1/Ifh1 pair. Rapid depletion experiments allowed us to define a hierarchy of TF binding in which Rap1 acts as a pioneer factor required for binding of all other TFs. We also uncovered unexpected features underlying recruitment of Fhl1, whose forkhead DNA-binding domain is not required for binding at most promoters, and Hmo1, whose binding is supported by repeated motifs. Finally, we describe unusually micrococcal nuclease (MNase)-sensitive nucleosomes at all RPG promoters, located between the canonical +1 and -1 nucleosomes, which coincide with sites of Fhl1/Ifh1 and Hmo1 binding. We speculate that these "fragile" nucleosomes play an important role in regulating RPG transcriptional output.

A chemostat array enables the spatio-temporal analysis of the yeast proteome.

Observing cellular responses to perturbations is central to generating and testing hypotheses in biology. We developed a massively parallel microchemostat array capable of growing and observing 1,152 yeast-GFP strains on the single-cell level with 20 min time resolution. We measured protein abundance and localization changes in 4,085 GFP-tagged strains in response to methyl methanesulfonate and analyzed 576 GFP strains in five additional conditions for a total of more than 10,000 unique experiments, providing a systematic view of the yeast proteome in flux. We observed that processing bodies formed rapidly and synchronously in response to UV irradiation, and in conjunction with 506 deletion-GFP strains, identified four gene disruptions leading to abnormal ribonucleotide-diphosphate reductase (Rnr4) localization. Our microchemostat platform enables the large-scale interrogation of proteomes in flux and permits the concurrent observation of protein abundance, localization, cell size, and growth parameters on the single-cell level for thousands of microbial cultures in one experiment.

KRAB-zinc finger proteins and KAP1 can mediate long-range transcriptional repression through heterochromatin spreading.

Krüppel-associated box domain-zinc finger proteins (KRAB-ZFPs) are tetrapod-specific transcriptional repressors encoded in the hundreds by the human genome. In order to explore their as yet ill-defined impact on gene expression, we developed an ectopic repressor assay, allowing the study of KRAB-mediated transcriptional regulation at hundreds of different transcriptional units. By targeting a drug-controllable KRAB-containing repressor to gene-trapping lentiviral vectors, we demonstrate that KRAB and its corepressor KAP1 can silence promoters located several tens of kilobases (kb) away from their DNA binding sites, with an efficiency which is generally higher for promoters located within 15 kb or less. Silenced promoters exhibit a loss of histone H3-acetylation, an increase in H3 lysine 9 trimethylation (H3K9me3), and a drop in RNA Pol II recruitment, consistent with a block of transcriptional initiation following the establishment of silencing marks. Furthermore, we reveal that KRAB-mediated repression is established by the long-range spreading of H3K9me3 and heterochromatin protein 1 beta (HP1beta) between the repressor binding site and the promoter. We confirm the biological relevance of this phenomenon by documenting KAP1-dependent transcriptional repression at an endogenous KRAB-ZFP gene cluster, where KAP1 binds to the 3' end of genes and mediates propagation of H3K9me3 and HP1beta towards their 5' end. Together, our data support a model in which KRAB/KAP1 recruitment induces long-range repression through the spread of heterochromatin. This finding not only suggests auto-regulatory mechanisms in the control of KRAB-ZFP gene clusters, but also provides important cues for interpreting future genome-wide DNA binding data of KRAB-ZFPs and KAP1.

A synthetic low-frequency mammalian oscillator.

Circadian clocks have long been known to be essential for the maintenance of physiological and behavioral processes in a variety of organisms ranging from plants to humans. Dysfunctions that subvert gene expression of oscillatory circadian-clock components may result in severe pathologies, including tumors and metabolic disorders. While the underlying molecular mechanisms and dynamics of complex gene behavior are not fully understood, synthetic approaches have provided substantial insight into the operation of complex control circuits, including that of oscillatory networks. Using iterative cycles of mathematical model-guided design and experimental analyses, we have developed a novel low-frequency mammalian oscillator. It incorporates intronically encoded siRNA-based silencing of the tetracycline-dependent transactivator to enable the autonomous and robust expression of a fluorescent transgene with periods of 26 h, a circadian clock-like oscillatory behavior. Using fluorescence-based time-lapse microscopy of engineered CHO-K1 cells, we profiled expression dynamics of a destabilized yellow fluorescent protein variant in single cells and real time. The novel oscillator design may enable further insights into the system dynamics of natural periodic processes as well as into siRNA-mediated transcription silencing. It may foster advances in design, analysis and application of complex synthetic systems in future gene therapy initiatives.

Mapping the fine structure of a eukaryotic promoter input-output function.

The precise tuning of gene expression levels is essential for the optimal performance of transcriptional regulatory networks. We created 209 variants of the Saccharomyces cerevisiae PHO5 promoter to quantify how different binding sites for the transcription factor Pho4 affect its output. We found that transcription-factor binding affinities determined in vitro could quantitatively predict the output of a complex yeast promoter. Promoter output was precisely tunable by subtle changes in binding-site affinity of less than 3 kcal mol(-1), which are accessible by modifying 1-2 bases. Our results provide insights into how transcription-factor binding sites regulate gene expression, their possible evolution and how they can be used to precisely tune gene expression. More generally, we show that in vitro binding-energy landscapes of transcription factors can precisely predict the output of a native yeast promoter, indicating that quantitative models of transcriptional regulatory networks are feasible.

A synthetic metabolite-based mammalian inter-cell signaling system.

Functionally well-characterized modular transcription units represent the genetic repertoire for the design of synthetic gene networks operating inside individual mammalian cells. Interconnection of specialized cells to multicellular assemblies that could execute complex computational functions requires synthetic signaling systems, which process and synchronize metabolic information between mammalian cells. In this study we have designed a metabolite-controlled inter-cellular signaling device consisting of a human sender cell line stably engineered for constitutive expression of the human liver-type arginase and a transgenic receiver cell line harboring a synthetic circuit, which produced a human glycoprotein in response to L-arginine levels in the culture medium. Quantitative characterization of the system components enabled precise prediction of l-arginine degradation and product gene expression kinetics and showed that two independent transgenic cell lines could functionally inter-operate to form a metabolite-controlled device which is able to precisely time desired target gene expression. Synthetic gene circuits modulating the transfer of metabolic information from a sender to a receiver cell line may enable the design of synthetic hormone systems supporting communication across multicellular assemblies.