Guest Speaker – Physical (CHEM 781) – April 19, 2023

Bio
Dr. Jian Liu graduated from Peking University with B.S. in Chemistry in 2000 and earned his Ph.D. in Theoretical Chemistry from the University of California, Berkeley in 2005. He completed postdoctoral fellowships at the University of California, San Diego, Center for Theoretical Biological Physics from 2005 to 2007 and at the University of California, Berkeley, from 2007 to 2009. Dr. Liu joined the NIH as a Principal Investigator in 2010 and moved to Johns Hopkins School of Medicine as an Associate Professor in 2019. Dr. Liu takes a distinct approach to theoretical biology, treating cellular systems as discrete functional modules comprising a set of critical players. This allows both simplification and retention of essential biological features. The larger goal of this modular approach is to allow for processes to be combined at a theoretical level to reveal the interplay among them in the cell as a whole.

Abstract
In bacteria, most low-copy-number plasmid and chromosomally encoded partition systems belong to the tripartite ParABS partition machinery. Despite the importance in genetic inheritance, the mechanisms of ParABS-mediated genome partition are not well understood. Combining theory and experiment, we provided evidence that the ParABS system—DNA partitioning in vivo via the ParA-gradient-based Brownian ratcheting—operates near a transition point in parameter space (i.e., a critical point), across which the system displays qualitatively different motile behaviors. This near-critical-point operation adapts the segregation distance of replicated plasmids to the half length of the elongating nucleoid, ensuring both cell halves to inherit one copy of the plasmids. Further, we demonstrated that the plasmid localizes the cytoplasmic ParA to buffer the partition fidelity against the large cell-to-cell fluctuations in ParA level. The spatial control over the near-critical-point operation not only ensures both sensitive adaptation and robust execution of partitioning but also sheds light on the fundamental question in cell biology: how do cells faithfully measure cellular-scale distance by only using molecular-scale interactions?