Xiao Group

         The overall objective of our research is to study the dynamics of cellular processes as they occur in real time at the single-molecule and single-cell level. The depth and breadth of our research require an interdisciplinary approach, combining biological, biochemical and biophysical methods to quantitatively address compelling biological problems. With unprecedented sensitivities to detect individual molecules, the use of single-molecule and single-cell approaches allows one to access information that is not readily available to traditional ensemble measurements. For example, one can explore heterogeneities among the different molecules and cells within a population and, more importantly, track motions of individual molecules and their biochemical interactions. These are particularly suitable for illustrating the mechanisms of many cellular processes resulting from highly dynamic interactions among proteins, DNAs and small molecules, which are not usually present in large copy numbers inside the cells. We are currently focusing on the following projects:

Dynamics and structure of the E. coli division complex
         Cell division is essential for the survival and development of all organisms. In E. coli, at least ten essential proteins assemble at the midcell to form a ring-like division complex, the divisome, in an ordered fashion to carry out cytokinesis. We are interested in knowing how this complex is orchestrated to function and regulated to coordinate with other essential cellular events. Specifically, we focus on the following questions: When does each division protein localize to the midcell during the cell cycle? How does each division protein move to the midcell to assemble into the divisome? Where does each division protein locate within the divisome? We will address these questions for ten essential division proteins employing highly sensitive single-molecule fluorescence detection and live-cell imaging.

Noise control mechanisms in gene regulatory networks
         Gene expression is stochastic in nature as the components involved exist in small copy numbers. Such stochasticity inevitably leads to output noise. However, "Noisy gene expression" is intuitively at odds with the reliable formation of precise gene expression patterns cells and organisms exhibit during development and growth. We wonder: how do cells function with amazing precisions and robustness when the underlying molecular events are inherently stochastic? To answer this question, we have developed single-molecule gene expression fluorescence reporters that allow us to directly monitor the production of single protein molecules in real time. We are currently using the genetic switch of phage as a model system to investigate the noise control mechanisms of gene regulatory networks. Specifically, we ask: does the overall regulatory architecture of the network, other than the fine-tuning of specific regulatory parameters, play an indispensable role in controlling noise in gene expression?

         We are also interested in developing better fluorescent reporters and single pair fluorescent resonance energy transfer (spFRET) reporters to allow the probing of fast kinetics of cellular processes and interactions among protein complexes. The use of single molecule fluorescence microscopy methods, in combination with statistical analysis will not only complement traditional population studies, but also shed new lights on the mechanisms of these cellular processes at an unprecedented level. The methodology developed in the research will open a new dimension in characterizing biological systems in live cells.