Der eukaryotische Zellzyklus ist ein streng regulierter Prozess, für dessen zeitlichen Ablauf unter anderem oszillierende Genexpression notwendig ist. Die Regulation und die zeitliche Koordination des Zellzyklus sind nach wie vor fundamentale Fragen der Zellbiologie. Spezifische Ereignisse, wie DNA Replikation und Zellkernteilung, können vier Zellzyklusphasen zugeordnet werden, welche durch Cyclin-abhängige Kinasen, Cycline und deren Inhibitoren reguliert werden. Während in Saccharomyces cerevisiae Cyclin-abhängige Kinasen (Cdc28, Pho85) über den gesamten Zellzyklus zu Verfügung stehen, werden Cycline und ihre Inhibitoren nur in spezifischen Phasen exprimiert. In S. cerevisiae sind drei wichtige G1-Cycline (Cln1-Cln3) in die oszillierende Genexpression involviert. In dieser Arbeit wurde die zeitaufgelöste, transkriptomweite Genexpression im Wildtyp und in Cyclindeletionsmutanten gemessen. Um die Rolle der G1-Cycline für die Feinabstimmung des Zellzykluses zu verstehen, wurden Gene nach charakteristischen Expressionsprofilen geclustert, Expressionsmaxima detektiert, ein Transkriptionsfaktornetzwerk integriert und Zellzyklusphasendauern bestimmt. Um Unterschiede zwischen der Rolle der Cycline zu verstehen, wurden die Zellen zusätzlich Osmostress ausgesetzt. Des Weiteren wurde mit Hilfe von RNA-Fluorescence In Situ Hybridization (FISH) die Expression zweier Cycline (PCL1 und PCL9), die an Pho85 binden, auf Einzelzellniveau gemessen. Um die Expression in spezifischen Zellzyklusphasen zu quantifizieren, wurden einzelne Zellen mithilfe von Zellzyklusmarkern spezifischen Zellzyklusphasen zugeordnet. Nachdem die Expression unter normalen Wachstumsbedingungen gemessen wurde, wurde zusätzlich Osmostress angewandt. Durch die Kombination einer Einzelzellquantifizierung und einer transkriptomweiten Methode konnten spezifische Aufgaben der Cycline, Cln1, Cln2 und Cln3, erforscht werden. Zusätzlich konnten backup Mechanismen für die Zellzyklusregulation entschlüsselt werden. / The eukaryotic cell cycle is a highly ordered process. For its timing and progression, oscillating gene expression is crucial. The stability of cell cycle regulation and the exact timing is still a fundamental question in cell biology. Specific events, like DNA replication and nuclear division can be assigned to four distinct phases. These events are regulated by cyclin-dependent kinases, cyclins and their inhibitors. In Saccharomyces cerevisiae cyclin-dependent kinases (Cdc28, Pho85) are present throughout the cell cycle, while cyclins and their inhibitors are only expressed and active during specific phases. The G1 cyclins Cln1-3 are essential players to induce oscillating gene expression and are thereby involved in the fine-tuning of the cell cycle. To understand the role of the G1 cyclins for exact cell cycle timing and oscillating gene expression, time-resolved, transcriptome-wide gene expression in wild type and cyclin deletion mutants were measured. Characteristic expression profiles were clustered, precise peak times for each gene were estimated, a transcription factor network was integrated and cell cycle phase durations were defined. To further understand the role and differences of each cyclin osmostress was applied. Furthermore the expression of two cyclins (PCL1 and PCL9) corresponding to the cyclin-dependent kinase Pho85 was measured in single cells. Using RNA-Fluorescence In Situ Hybridization (FISH) and cell cycle progression markers, high and low expression phases and absolute numbers of mRNAs were obtained. Gene expression was quantified under normal and osmostressed growth conditions to understand the necessity of the cyclins for osmostress adaptation in different cell cycle phases. By the combination of a single cell and a transcriptome-wide approach distinct roles of G1 cyclins Cln1, Cln2 and Cln3 were deciphered and an insight in the backup mechanisms during cell cycle progression for normal and osmostressed growth conditions were proposed.

Título de Ingeniería en Biotecnología Molecular / Los mecanismos involucrados en el control post-transcripcional del ciclo replicativo del Virus de la Inmunodeficiencia Humana (VIH), específicamente los eventos moleculares que permiten la interacción del ARN genómico (ARNg) viral con la maquinaria celular para su transporte, traducción o empaque dentro de la célula, aún no han sido completamente dilucidados. Actualmente existen diversas técnicas para el estudio de la localización sub-celular de ARNg, entre las que destaca RNA FISH (RNA Fluorescent in situ hybridization), método ampliamente utilizado para el estudio de la localización y cambios temporales de ARN y ribonucleoproteínas. Generalmente, en esta técnica se utilizan sondas acopladas a fluoróforos orgánicos que hibridan a regiones específicas para las que han sido diseñadas. No obstante, estas sondas presentan fotoblanqueamiento significativo, y un espectro de emisión amplio (50-100 nm), lo que limita su uso. Es por esto que surge la necesidad de recurrir a nuevas alternativas, tales como sondas acopladas a Quantum dots (QDs), los cuales en contraste con fluoróforos orgánicos poseen una vida fluorescente significativamente más larga, mayor fotoestabilidad y un amplio espectro de absorción. Considerando lo anterior, en el presente trabajo se propone la construcción de sondas específicas asociadas a QDs compuestos de Cadmio-Telurio recubiertos por Glutatión (QDs CdTe- GSH) que reconocen la región pBSK-GagPol como matriz para la transcripción in vitro, obteniendo así fragmentos de ARN, los cuales fueron desfosforilados en su extremo 5', para luego incorporarles en su lugar un fosfato γ unido a un grupo sulfhidrilo (SH). De esta forma, los transcritos de ARN fueron capaces de unirse a QDs CdTe-GSH a través de enlaces disulfuro. Generando así sondas de ARN xiii acoplada a QDs CdTe-GSH capaces de unirse al ARNg de VIH-1 en presencia y ausencia de la proteína Rev,la cual actúa como un regulador clave en el control post-transcripcional de la expresión génica viral, actuando principalmente en los procesos de exportación nuclear y traducción. De manera que en estas condiciones fue posible validar a través de experimentos de RNA FISH, el ingreso citoplasmático y nuclear de la sonda de ARN acoplada a QDs CdTe-GSH, además de una interacción específica con el ARN genómico de VIH-1. Este hecho representa la prueba de concepto de que es posible generar una sonda acoplada a QDs específica contra el ARN genómico de VIH-1, permitiendo el estudio de la expresión génica del virus, lo que también abre nuevas posibilidades para el estudio de VIH-2 u otros tipos de virus. / The mechanisms involved in the post-transcriptional control of the replicative cycle of the Human Immudeficiency Virus (HIV), specifically the molecular events allowing the interaction between the viral genomic RNA (gRNA) and the cellular machinery for the transport, translation or packaging, have not been elucidated yet. Currently, the study of localization and temporary changes of RNA and ribonucleoproteins relies mainly on RNA FISH (RNA Fluorescent in situ hybridization)-based strategies. RNA FISH uses specific hybridization probes coupled to organic fluorophores. However, these fluorescent molecules commonly present limiting characteristics such as significant photobleaching and a wide emission spectrum (50-100 nm). Therefore, a considerable demand arises for new alternatives, such as probed coupled to Quantum dots (QDs), which in contrast to organic fluorophores, exhibit longer fluorescence lifetime, higher photostability and broad absorption spectra. Considering previous facts, the aim of this work was to develop specific probes coupled to glutathione-capped cadmium-telluride quantum dots (QDs CdTe-GSH), able of recognizing and associate with the Gag-Pol region present on the HIV-1 genomic RNA (gRNA). Thus, in order to achieve this objective, the vector pBSK-GagPol was used as a template for in vitro transcription of Gag-Pol complementary RNA, which was fragmented and dephosphorylated at the 5' end, with the purpose to incorporate a γ phosphate coupled to a sulfhydryl group (SH) instead. Then, the SH-containing RNA fragments were attached to QDs CdTe-GSH by a disulfide bond. As a result, we generated single-stranded RNA probes coupled to QDs CdTe-GSH capable of hybridizing to HIV-1 gRNA in the presence and absence of the viral protein Rev, which acts as a key regulator of the post-transcriptional control of viral gene expression acting mainly during nuclear export and translation. Thereby, under these conditions, we validated the cytoplasmic and nuclear entrance of the probe coupled to Qds CdTe-GSH, and specific interaction between the probe and gRNA of HIV-1. This fact represents the proof of concept that it is possible to generate RNA probes coupled to QDs CdTe-GSH to study genetic expression of HIV-1, and also opens up new opportunities for the study of HIV-2 or other virus types.

Die Genexpression ist ein stochastischer Prozess, dessen strenge Regulation einen ungestörten Zellzyklusverlauf ermöglicht. Jeglicher Stress löst eine Neuprogrammierung der Expression und somit einen Stillstand des Zellzyklus aus. Um ein besseres Verständnis des eukaryotischen Zellzyklus zu erlangen, wurde in dieser Arbeit die Fluoreszenzmikroskopie einzelner Zellen (S.cerevisiae) mit stochastischer Modellierung der Hauptregulatorgene des G1/S-Übergangs (SIC1, CLN2, CLB5) kombiniert. Mithilfe des MS2-CP-Systems wurden mRNA-Level von SIC1 in lebenden Zellen bestimmt und verschiedene Transportwege von SIC1-mRNA visualisiert. RNA-FISH in Kombination mit genetischen und morphologischen Markierungen ermöglichte es, die absolute Quantifizierung von SIC1-, CLN2- und CLB5-mRNA in allen Zyklusphasen vorzunehmen. Die Auswirkung von Osmostress, in Hinblick auf eine transkriptionale Verzerrung, wurde untersucht. Basierend auf den experimentellen-Daten wurde ein stochastisches Model entwickelt, dass die Expression von SIC1, CLN2 und CLB5 mRNA und Proteinlevel in Abhängigkeit von Osmostress über den gesamten Zellzyklus hinweg abbildet. Die Modellierung ermöglichte eine in silico Synchronisation und somit die Extraktion kinetischer Parameter. Die Expression der beobachteten Gene wurde im Verlauf des Zellzyklus nicht ein- und ausgeschaltet, stattdessen kam es zu Phasen hoher oder niedriger Expression. Niedriger SIC1 Expression gewährleistete niedriger Sic1 Protein Verzerrung und robustes G1/S Timing. CLN2 und CLB5 zeigten ein maximales Expressionslevel in G1 und auch eine erhöhte Expression in der späten Mitose. Osmostress induzierte einen langanhaltenden Effekt auf die Transkription und die Dauer der Zellzyklusphasen. Der hier vorgestellte Ansatz ermöglichte quantitative Einblicke in die Genexpression und zeitliche Koordination des Zellzyklus von S.cerevisiae. Einige der hier beobachteten Regulationsmechanismen könnten allgemeine Gültigkeit im eukaryotischen Zellzyklus besitzen. / Gene expression is a stochastic process and its appropriate regulation is critical for cell cycle progression. Cellular stress response requires expression reprogramming and cell cycle arrest. Time-resolved quantitative methods on single cells are needed to understand eukaryotic cell cycle in context of noisy gene expression and external perturbations. We applied single-cell fluorescence microscopy and stochastic modeling to SIC1, CLN2 and CLB5, the main G1/S regulators in S. cerevisiae. Using MS2-CP system we estimated SIC1 mRNA levels and visualized different types of transport for SIC1 mRNA particles in living cells. With RNA-FISH combined to genetic and morphological markers we monitored absolute numbers of mRNA and transcriptional noise over cell cycle phases with and without osmostress. Stochastic modeling enabled in silico synchronization, the extraction of kinetic parameters as well as expanded the static mRNA data into time courses for mRNAs, proteins and their noise. Based on our experimental data we developed a stochastic model of G1/S timing centered on SIC1 and a second one for the entire cell cycle involving SIC1, CLN2 and CLB5 and the response to osmostress. All three genes exhibited basal expression throughout cell cycle enlightening that transcription is not divided in on and off but rather in high and low phases. A low SIC1 transcript level ensured a low protein noise and a robust timing of the G1/S transition. CLN2 and CLB5 showed main expression peaks in G1 as well as an expression upshift in late mitosis. Osmostress induced different periods of transcriptional inhibition for CLN2 and CLB5 and long-term impact on cell cycle phase duration. Our approach disclosed detailed quantitative insights into gene expression and cell cycle timing, not available from bulk experiments. Importantly some regulation mechanisms specific to SIC1, CLN2 and CLB5 might be generalized to other genes as well as to other organisms.

In bacteria, protein synthesis can occur tightly coupled to transcription. In eukaryotes, it is believed that translation occurs solely in the cytoplasm; I test whether some occurs in nuclei and find: (1) L-azidohomoalanine (Aha) – a methionine analogue (detected by microscopy after attaching a fluorescent tag using ‘click’ chemistry) – is incorporated within 5 s into nuclei in a process sensitive to the translation inhibitor, anisomycin. (2) Puromycin – another inhibitor that end-labels nascent peptides (detected by immuno-fluorescence) – is similarly incorporated in a manner sensitive to a transcriptional inhibitor. (3) CD2 – a non-nuclear protein – is found in nuclei close to the nascent RNA that encodes it (detected by combining indirect immuno-labelling with RNA fluorescence in situ hybridization using intronic probes); faulty (nascent) RNA is destroyed by a quality-control mechanism sensitive to translational inhibitors. I conclude that substantial translation occurs in the nucleus, with some being closely coupled to transcription and the associated proof-reading. Moreover, most peptides made in both the nucleus and cytoplasm are degraded soon after they are made with half-lives of about one minute. I also collaborated on two additional projects: the purification of mega-complexes (transcription ‘factories’) containing RNA polymerases I, II, or III (I used immuno-fluorescence to confirm that each contained the expected constituents), and the demonstration that some ‘factories’ specialize in transcribing genes responding to tumour necrosis factor α – a cytokine that signals through NFκB (I used RNA fluorescence in situ hybridization coupled with immuno-labelling to show active NFκB is found in factories transcribing responsive genes).