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Khalil, A. S.

Publications and source records attributed to Khalil, A. S..

3 recordsLinked to original sources

A generalizable experimental framework for automated cell growth and laboratory evolution

In the post-genomics era, exploration of phenotypic adaptation is limited by our ability to experimentally control selection conditions, including multi-variable and dynamic pressure regimes. While automated cell culture systems offer real-time monitoring and fine control over liquid cultures, they are difficult to scale to high-throughput, or require cumbersome redesign to meet diverse experimental requirements. Here we describe eVOLVER, a multipurpose, scalable DIY framework that can be easily configured to conduct a wide variety of growth fitness experiments at scale and cost. We demonstrate eVOLVERs versatility by configuring it for diverse growth and selection experiments that would be otherwise challenging for other systems. We conduct high-throughput evolution of yeast across different population density niches. We perform growth selection on a yeast knockout library under temporally varying temperature regimes. Finally, inspired by large-scale integration in electronics and microfluidics, we develop novel millifluidic multiplexing modules that enable complex fluidic routines including multiplexed media routing, cleaning, vial-to-vial transfers, and automated yeast mating. We propose eVOLVER to be a versatile design framework in which to study, characterize, and evolve biological systems.

systems biology

Hsf1 phosphorylation generates cell-to-cell variation in Hsp90 levels and promotes phenotypic plasticity

Clonal populations of cells exhibit cell-to-cell variation in the transcription of individual genes. In addition to this \"noise\" in gene expression, heterogeneity in the proteome and the proteostasis network expands the phenotypic diversity of a population. Heat shock transcription factor (Hsf1) regulates chaperone gene expression, thereby coupling transcriptional noise to proteostasis. Here we show that cell-to-cell variation in Hsf1 activity is an important determinant of phenotypic plasticity. Budding yeast cells with high Hsf1 activity were enriched for the ability to acquire resistance to an antifungal drug, and this enrichment depended on Hsp90 - a known \"phenotypic capacitor\" and canonical Hsf1 target. We show that Hsf1 phosphorylation promotes cell-to-cell variation, and this variation - rather than absolute Hsf1 activity - promotes antifungal resistance. We propose that Hsf1 phosphorylation enables differential tuning of the proteostasis network in individual cells, allowing populations to access a wide range of phenotypic states.

systems biology

Hsf1 and Hsp70 constitute a two-component feedback loop that regulates the yeast heat shock response

Models for regulation of the eukaryotic heat shock response typically invoke a negative feedback loop consisting of the transcriptional activator Hsf1 and a molecular chaperone encoded by an Hsf1 target gene. Previously, we identified Hsp70 as the chaperone responsible for Hsf1 repression in Saccharomyces cerevisiae and constructed a mathematical model based on Hsp70-mediated negative feedback that recapitulated the dynamic activity of Hsf1 during heat shock. The model was based on two assumptions: dissociation of Hsp70 activates Hsf1, and transcriptional induction of Hsp70 deactivates Hsf1. Here we validated these assumptions. First, we severed the feedback loop by uncoupling Hsp70 expression from Hsf1 regulation. As predicted by the model, Hsf1 was unable to efficiently deactivate in the absence of Hsp70 transcriptional induction. Next we mapped a discrete Hsp70 binding site on Hsf1 to a motif in the C-terminal activation domain known as conserved element 2 (CE2). Removal of CE2 resulted in increased Hsf1 activity under non-heat shock conditions and delayed deactivation kinetics. In addition, we uncovered a role for the N-terminal domain of Hsf1 in negatively regulating DNA binding. These results reveal the quantitative control mechanisms underlying the feedback loop charged with maintaining cytosolic proteostasis.

biochemistry