Assistant Professor
Department of Biophysics and Biophysical Chemistry
School of Medicine

xiao@jhmi.edu

708A WBSB
725 N. Wolfe Street
Baltimore, MD 21205

Office: 410-614-0338
Lab: 410-614-1760

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 of the E. coli cell division complex assembly
We are interested in knowing how this complicated process, involving more than ten different proteins and consisting of thousands copies of protein unites, is orchestrated to function. We are taking a systematic approach, starting with three early division proteins FtsZ, FtsA and ZipA and soon expanding to the rest of the gang, to study their spatial organizations, temporal arrivals and correlation of expressions during the assembly of the divisome in individual live E. coli cells. We employ highly sensitive single-molecule fluorescence microscopy to track individual protein molecules and monitor their dynamics as the process occurs in real time.

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.

Selected Publications
Xiao, J., J. Elf, G. Li, J. Yu, and X.S. Xie. (2007) Imaging gene expression in living cells at the single molecule level. In Single Molecules: A Laboratory Manual, ed. by Selvin, and Ta. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. (in press)

Singleton, S.F., A.I. Roca, A.M. Lee, and J. Xiao. (2007) Probing the structure of RecA-DNA filaments. Advantages of a fluorescent guanine analog. Tetrahedron 63:3553-3566.

Yu*, J., Xiao*, J., X. Ren, K. Lao, and X.S. Xie. (2006) Probing gene expression in live E. coli cells: one molecule at a time. Science 311:1600-1603.
*first two authors contributed equally to the work.

Xiao, J., A. Lee, and and S.F. Singleton. (2006) Direct evaluation of a kinetic model for RecA-mediated DNA-strand exchange: the importance of nucleic acid dynamics and entropy during homologous genetic recombination. ChemBioChem. 7:1265-1278.

Xiao, J., A. Lee, and S.F. Singleton. (2006) Construction and evaluation of a kinetic scheme of RecA mediated DNA strand exchange. Bipolymers. 81:473-496.

Lee, A., J. Xiao, and S.F. Singleton. (2006) Origins of sequence selectivity in homologous genetic recombination: insights from rapid kinetic probing of RecA-mediated DNA strand exchange. J. Mol. Biol. 360:343-359.

Xiao, J., and S.F. Singleton. (2002) Elucidating a key intermediate in homologous DNA strand exchange: structural characterization of the RecA-triple-stranded DNA complex using fluorescence resonance energy transfer. J. Mol. Biol. 320:529-558.

Singleton, S.F., and J. Xiao. (2001-2002) The stretched DNA geometry of recombination and repair nucleoprotein filaments. Bipolymers. 61:145-158.

Martin, S.R., A.Q. Lu, J. Xiao, J. Kleinjung, K. Beckingham, and P.M. Bayley. (1999) Conformational and metal-binding properties of androcam, a testis-specific calmodulin-related protein from Drosophila. Protein Sci. 8:2444-2454.