Browsing by Author "Narula, Jatin"

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    Chromosomal Arrangement of Phosphorelay Genes Couples Sporulation and DNA Replication
    (Elsevier, 2015) Narula, Jatin; Kuchina, Anna; Lee, Dong-yeon; Fujita, Masaya; Süel, Gürol M.; Igoshin, Oleg A.
    Genes encoding proteins in a common regulatory network are frequently located close to one another on the chromosome to facilitate co-regulation or couple gene expression to growth rate. Contrasting with these observations, here, we demonstrate a functional role for the arrangement of Bacillus subtilis sporulation network genes on opposite sides of the chromosome. We show that the arrangement of two sporulation network genes, one located close to the origin and the other close to the terminus, leads to a transient gene dosage imbalance during chromosome replication. This imbalance is detected by the sporulation network to produce cell-cycle coordinated pulses of the sporulation master regulator Spo0A∼P. This pulsed response allows cells to decide between sporulation and continued vegetative growth during each cell cycle spent in starvation. The simplicity of this coordination mechanism suggests that it may be widely applicable in a variety of gene regulatory and stress-response settings.
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    Design Principles of Cellular Differentiation Regulatory Networks
    (2015-12-01) Narula, Jatin; Igoshin, Oleg A; Tabor, Jeffrey J; Bennett, Matthew R
    To understand cellular differentiation programs is to understand the often large and complex gene regulatory networks (GRNs) that control and orchestrate these programs. The work presented here aims to exploit the wealth of newly available experimental information and methods to identify design principles that relate how GRN structures relate to the functional requirements in three model differentiation/stress-response programs: embryonic hematopoiesis, sporulation and σB general stress response. First we used a statistical thermodynamic approach to characterize the biophysical mechanisms of combinatorial regulation by distant enhancers in eukaryotes and demonstrate how the GRN controlling embryonic hematopoiesis acts as an irreversible bistable switch with low-pass noise filtering properties. We further used our model of the hematopoiesis network to reconcile discrepant experimental observations about the regulator Runx1 and explained how it limits HSC emergence in vitro. In the second project we investigated the Bacillus subtilis sporulation network and showed how a cascade of feed-forward loops downstream of the master regulatory Spo0A~P control cell-fate during starvation. We also identified a rate-responsive network module in the Spo0A regulon to explain why accelerated accumulation of Spo0A~P leads to a dramatic reduction in sporulation efficiency. Further we found that the arrangement of two sporulation network genes on opposite ends of the chromosome ties Spo0A~P activation to the DNA replication status. We were also able to show that the slowdown of cell growth is the primary starvation signal that determines sporulation cell-fates by controlling Spo0A~P activation. For the third project we built a detailed model of the σB network in Bacillus subtilis to mechanistically explain the experimentally observed pulsatile response of this network under stress. We further showed that the same network architecture that enables this pulsatile response insulates the σB network from the effects of competition for cellular resources like RNA polymerase. The design principles identified in the studies of these networks are related to their topological structure and function rather than the specific genes and proteins that comprise them. As a result, we expect them to be widely applicable to and help in the study of a diverse array of other differentiation GRNs.
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    Slowdown of growth controls cellular differentiation
    (EMBO Press, 2016) Narula, Jatin; Kuchina, Anna; Zhang, Fang; Fujita, Masaya; Süel, Gürol M.; Igoshin, Oleg A.
    How can changes in growth rate affect the regulatory networks behavior and the outcomes of cellular differentiation? We address this question by focusing on starvation response in sporulating Bacillus subtilis. We show that the activity of sporulation master regulator Spo0A increases with decreasing cellular growth rate. Using a mathematical model of the phosphorelay—the network controlling Spo0A—we predict that this increase in Spo0A activity can be explained by the phosphorelay protein accumulation and lengthening of the period between chromosomal replication events caused by growth slowdown. As a result, only cells growing slower than a certain rate reach threshold Spo0A activity necessary for sporulation. This growth threshold model accurately predicts cell fates and explains the distribution of sporulation deferral times. We confirm our predictions experimentally and show that the concentration rather than activity of phosphorelay proteins is affected by the growth slowdown. We conclude that sensing the growth rates enables cells to indirectly detect starvation without the need for evaluating specific stress signals.
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    Ultrasensitivity of the Bacillus subtilis sporulation decision
    (National Academy of Sciences, 2012) Narula, Jatin; Devi, Seram N.; Fujita, Masaya; Igoshin, Oleg A.
    Starving Bacillus subtilis cells execute a gene expression program resulting in the formation of stress-resistant spores. Sporulation master regulator, Spo0A, is activated by a phosphorelay and controls the expression of a multitude of genes, including the forespore- specific sigma factor σF and the mother cell-specific sigma factor σE. Identification of the system-level mechanism of the sporulation decision is hindered by a lack of direct control over Spo0A activity. This limitation can be overcome by using a synthetic system in which Spo0A activation is controlled by inducing expression of phosphorelay kinase KinA. This induction results in a switch-like increase in the number of sporulating cells at a threshold of KinA. Using a combination of mathematical modeling and single-cell microscopy, we investigate the origin and physiological significance of this ultrasensitive threshold. The results indicate that the phosphorelay is unable to achieve a sufficiently fast and ultrasensitive response via its positive feedback architecture, suggesting that the sporulation decision is made downstream. In contrast, activation of σF in the forespore and of σE in the mother cell compartments occurs via a cascade of coherent feed-forward loops, and thereby can produce fast and ultrasensitive responses as a result of KinA induction. Unlike σF activation, σE activation in the mother cell compartment only occurs above the KinA threshold, resulting in completion of sporulation. Thus, ultrasensitive σE activation explains the KinA threshold for sporulation induction. We therefore infer that under uncertain conditions, cells initiate sporulation but postpone making the sporulation decision to average stochastic fluctuations and to achieve a robust population response.