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Biophysics and Biophysical Chemistry
on using a single-molecule method to see how genes are regulated:
Your arrival here in 2006 marked a foray by the Program for Molecular Biophysics into cutting edge single-molecule research. How and when did you first venture into this forward-thinking field?
XIAO: I was studying biochemistry as a grad student when I first heard about single-molecule biophysics at a Biophysical Society annual meeting. I thought it was really cool that you could look at only one single molecule at a time and see what it is doing. At the time, I was trying to look at protein-DNA interactions in test tubes, which contain millions – billions of molecules. Like everyone else, I measured their properties under the assumption that they were all doing exactly the same thing. But I didn’t really know what was really happening in that tube of billions of individual molecules because molecules are dynamic and there are great heterogeneities in their properties. A measurement with a simple mean number does not necessarily represent all the things that are going on. When I gave a research talk to my department, I was asked how I knew that the property I measured in my test tube indeed came from one single reaction species. I couldn’t really answer the question without referring to many control experiments.
Was it that question alone that propelled you to explore the possibility of isolating single molecules of interest and watching how these molecules carry out important biological functions?
XIAO: That question made me wonder whether I should really try to see what my molecules were doing individually. Then, in 2002, at the Biophysical Society meeting in San Francisco, I met Professor Sunney Xie from Harvard, who would become my post-doc advisor. He was looking at my poster, and asked me, “Do you want to look at gene expression in real time?” That’s actually the question that got me hooked. Gene expression by itself is a single-molecule problem; a cell usually only has one or a few copies of a gene and many important, regulatory protein molecules expressed from a single gene exist in low copy numbers. It has been very difficult to detect these proteins, not even to mention to watch the expression process in real time in a living cell. The question was challenging but exciting. I joined his group in that instant.
In that instant, you became the only biologist post-doc in an otherwise hard-core physical chemistry lab, as you set out to do single-molecule work in a way it hadn’t been done before. Wasn’t that an intimidating prospect -- not to mention a risky career move -- for a budding biophysicist?
XIAO: I guess at that time I was too excited about learning something completely new to calculate the pros and cons. And to be honest, at that time I had not really thought about whether I should pursue an academic or industrial career. Sunney’s lab at that time was working extensively on in vitro single molecule studies using fancy lasers and microscopes that I had never touched. Venturing into single-molecule studies in living cells was a pioneering step that Sunney took in order to watch how molecules behave in their natural physiological context in a live cell. It did sound a little intimidating at the beginning but I am glad that I jumped in without thinking too much. Otherwise I would not be at a position I am today.
What is the focus of your research now?
XIAO: My students and I manipulate single molecules in living cells to investigate the molecular mechanisms of both gene regulation and cell division. Our overall objective is to study the dynamics of cellular processes as they occur in real time at the single-molecule and single-cell level. We try to track, locate and count single protein molecules as they move or are produced in a live cell. For example, we recently imaged the E. coli cell division ring with super resolution by locating individual protein molecules one at a time. We also followed the production of a transcription factor by counting the number of protein molecules expressed in real time and discovered a new type of transcription activation. New insight we generated from these types of experiments would be difficult or impossible with traditional measurements.
Please describe your microscope room, which houses a customized and complicated laser system – a place which your 5-year-old son, Leo, has aptly dubbed “the light saber room.”
XIAO: Most of the time, it’s dark, and you can the humming of the laser. A green or yellow sheen emanates from the microscopes that we use to excite fluorescent molecules in live bacteria cells so we can observe their motions. Often, there’s a clicking sound: It’s a mechanical shutter opening and closing to pass or block the lasers. Right now, we are running our experiments on bacteria cells prepared in a special chamber so that they can grow and multiply. While they are growing, we are looking at the fluorescently labeled protein molecules inside to find out what they are doing.
-Interviewed by Maryalice Yakutchik