There are a variety of proteins and enzymes that act at specific spots on DNA. For example, there are a whole assortment of proteins that recognize the promoter regions in front of genes. There are also enzymes involved in replication and repair. How do these proteins get to the right site on the DNA?
It has been known for some time that these proteins must slide along the DNA strand in some manner. The proteins find their intended targets much too quickly for them to simply be diffusing around the cell and randomly encountering sites along the DNA. In 2006, a team of Harvard scientists led by Sunney Xie showed that the proteins are in direct contact with the DNA. The question is, were the proteins sliding down parallel to the DNA strands, or were they actually coiling along the helix?
This problem was solved by Sunney Xie, in collaboration with scientists from Harvard University, the Indian Institute of Science and Brookhaven National Laboratory. His team used a fluorescent dye to tag individual protein molecules and then used a fluorescence microscope to observe those proteins as they found and bound to their DNA targets. The scientists could not actually watch the path of the proteins, but they could measure the speed of the interactions.
If the molecules had been sliding straight down parallel to the DNA strands, they would have moved faster than if they curved around the helix. As it turns out, the proteins slide around the DNA like on a spiral staircase.
Suddenly, it seems like a lot more fun to be a DNA-binding protein molecule!