Experimental evidence suggests that Casp3 activation may result in XIAP cleavage and/or degradation, although this seems to be a cell-type-specific phenomenon (see Discussion). In the following, we demonstrate that Casp3-induced XIAP degradation (‘inhibition of inhibition’ = positive circuit) does not result in physiologically relevant bistability in the absence of other feedback amplification ...

Bistable switches are widely used in synthetic biology to trigger cellular functions in response to environmental signals. All bistable switches developed so far, however, control the expression of target genes without access to other layers of the cellular machinery. Here, we propose a bistable switch to control the rate at which cells take up a metabolite from the environment. An uptake switc...

In the field of systems and synthetic biology there has been an increasing interest for the use of genetic circuits during the last decade. Several circuits have been successfully put together, many of which were based on models. During this thesis a model for a toggle switch was analysed both deterministically and stochastically. The HOM2-circuit approximation for a bistable tuneable switch fr...

Precise control of cell proliferation is fundamental to tissue homeostasis and differentiation. Mammalian cells commit to proliferation at the restriction point (R-point). It has long been recognized that the R-point is tightly regulated by the Rb-E2F signaling pathway. Our recent work has further demonstrated that this regulation is mediated by a bistable switch mechanism. Nevertheless, the es...

Synthetic gene circuits are often engineered by considering the host cell as an invariable 'chassis'. Circuit activation, however, may modulate host physiology, which in turn can substantially impact circuit behavior. We illustrate this point by a simple circuit consisting of mutant T7 RNA polymerase (T7 RNAP*) that activates its own expression in the bacterium Escherichia coli. Although activa...

After infecting a CD4 T cell, Human Immunodeficiency Virus (HIV) can either replicate and kill the cell or enter latency, a dormant state of the virus where viral gene-expression is turned OFF. This cell fate decision between viral replication and latency is controlled by the viral regulatory protein, Tat. This protein is known to activate its own production, creating a positive feedback circui...

Positive feedback and cooperativity in the regulation of gene expression are generally considered to be necessary for obtaining bistable expression states. Recently, a novel mechanism of bistability termed emergent bistability has been proposed which involves only positive feedback and no cooperativity in the regulation. An additional positive feedback loop is effectively generated due to the i...

Feedback loops are ubiquitous features of biological networks and can produce significant phenotypic heterogeneity, including a bimodal distribution of gene expression across an isogenic cell population. In this work, a combination of experiments and computational modeling was used to explore the roles of multiple feedback loops in the bimodal, switch-like response of the Saccharomyces cerevisi...

Xenopus oocyte maturation is an example of an all-or-none, irreversible cell fate induction process. In response to a submaximal concentration of the steroid hormone progesterone, a given oocyte may either mature or not mature, but it can exist in intermediate states only transiently. Moreover, once an oocyte has matured, it will remain arrested in the mature state even after the progesterone i...