IMAGING
Watching protein expression one molecule at a time Single-molecule imaging methods confirmed that gene expression is a random, bursty process In 2006, X. Sunney Xie of Harvard University accomplished a rare feat. His group published one paper in Nature and another in Science in the same week. Those papers described the first observations of protein production in live cells with single-mole-
then at Princeton, “had reported bursty gene expression at the RNA level, so it was nice seeing this at the protein level,” Ha says. In subsequent work reported in 2010, Xie’s group used similar single-molecule
from the supercoiling and uncoiling of the DNA molecule (Cell 2014, DOI: 10.1016/j. cell.2014.05.038). Those studies raised the question of how cells change their gene expression levels. “In principle, one can change the burst frequency or burst duration and achieve the same outcome,” Ha notes. “Studies by several laboratories suggest that the primary means is by changing burst frequency.” Fundamental studies on gene expression continue today. “Microscopic understanding of how genes can turn on and off in a stochastic manner is an exciting area of research and is likely to yield different answers in different systems,” Ha says. “There are a lot of single-molecule actions in cells,” Xie says. “The capability to monitor single Real-time single-molecule monitoring in three different cell lineages (indicated in far right panel) of molecules in individual a fluorescently labeled protein demonstrates that protein expression happens in bursts. The vertical cells for a long time with dotted lines indicate times at which cell division occurred. this kind of sensitivity allows us to probe those stocule sensitivity (Nature 2006, DOI: 10.1038/ methods to quantify the expression of more chastic, low-probability events that have nature04599; Science 2006, DOI: 10.1126/ than 1,000 proteins in Escherichia coli (Sciimportant biological consequences.” science.1119623). ence 2010, DOI: 10.1126/science.1188308). In the decade since Xie’s pair of papers, Xie and his coworkers used fluorescence Even in this system-wide study, protein “our ability to measure things in individmicroscopy to watch random protein proproduction was stochastic and bursty. ual cells has dramatically improved,” says duction under conditions in which gene ex- That work also showed that, in bacteria, Golding, who is now at Baylor College of pression is repressed. Those measurements the levels of a protein and its correMedicine. But the technology for proteins supported the idea that such production sponding RNA transcript are has lagged behind that for happens in sporadic bursts, rather than in a uncorrelated. nucleic acids, he adds. “We steady stream. More recently, Xie and his cannot do single-cell whole Prior to that work, “bursty gene exprescoworkers have shown that proteomes yet,” Golding says. sion at the protein level was inferred from this bursty behavior happens “Something that allows us to static snapshots of single cells, and models even for highly induced gene Watch a video do high-throughput detechad to be used to deduce the parameters of expression. With single-molthat shows bursty tion and labeling of whatever gene expression bursts,” says Taekjip Ha, a ecule and single-cell experprotein production proteins we want would really biophysicist at Johns Hopkins University. iments, they found that the in action at cenm. move us forward.”—CELIA The year before, Ido Golding, a biophysicist bursty gene expression stems ag/2016dir50. ARNAUD
52
C&EN | CEN.ACS.ORG | DECEMBER 12/19, 2016
CREDIT: SCIENCE
Video Online
