Learning biophysical principles ofcellular organization

PROJECT SUMMARY This project addresses emergent collective behavior in living systems at two different scales, communities of bacteria and intracellular biomolecular condensates: (1) Beyond living together, bacteria communicate with each other using a process known as quorum sensing that relies on the production, detection, and response to diffusible signal molecules called autoinducers. Quorum sensing provides bacteria with information about their local cell density and the species complexity of their environment. Bacteria use the information they gather to beneficially control collective behaviors. Quorum sensing has been extensively studied in the laboratory. My goal is to understand how quorum-sensing operates in natural environments. How do the complex spatial structure and temporal dynamics of bacterial communities affect the strategies by which bacteria obtain and exploit quorum-sensing information? How does the presence of other species influence quorum sensing? The latter question has been brought to the fore by the observation that both eukaryotic hosts and bacterial viruses (phage) participate in quorum-sensing conversations with bacteria. The ubiquity of quorum sensing in the bacterial world implies that answers to these questions, even in specific contexts, have the potential to provide broad insights into microbial collective behavior across species and environments. (2) Phase separation is an emerging organizing principle for intracellular biology. Processes that are now understood to exploit phase separation include storage of genetic material, gene expression, ribosome synthesis, signaling, stress response, and metabolism. While each phase-separating system has unique features, there are universal questions relevant to all such systems. E.g., what controls the number, size, and location of condensates within a cell? By what mechanism(s) are particular components included while others are excluded from condensates? How do cells orchestrate physical interactions between condensates and other cellular structures? To address these questions, I will focus on a well-suited model organism and system: the genetically tractable alga Chlamydomonas reinhardtii and its pyrenoid, a phase-separated organelle responsible for efficient carbon fixation. Key advantages of this system are that phase separation of the pyrenoid is driven by two well- characterized components, the condensate’s in vivo liquidity is reproduced in vitro with no energy source, in vivo assembly/disassembly is controllable by external cues, and the pyrenoid is a single body that is both large and stable enough to systematically investigate its functional interactions with other cellular components including a network of membrane tubules. I anticipate that by focusing on underlying biophysical mechanisms, the results of these studies will generalize to a wide range of phase-separated intracellular systems.