RNA polymerase has one job: to make RNA from DNA templates. Overall, it does a great job, except that it needs help with some small but important details, like figuring out where genes begin and separating the strands of a DNA double helix. Enter the preinitiation complex: an assembly of at least six different transcription factors that find a gene’s start site, pull the DNA strands apart, and put the polymerase in the right spot. To help scientists understand how these molecules work together, Eva Nogales, an HHMI investigator at the University of California, Berkeley, created snapshots of the complex as it readied DNA for transcription.
Using cryo-electron microscopy—a technique for viewing flash-frozen samples through a microscope—Nogales and her colleagues made the electron microscopy equivalent of a stop-motion movie. They first captured an image of the complex’s core on a strand of DNA. Then they added the remaining factors one by one, taking pictures of the growing complex after each addition. The resulting short film, described in a paper published March 28, 2013, in Nature, reveals just how the transcription factors work together.
|This movie shows the assembly of the human transcription pre-initiation complex as revealed by successive cryo-EM reconstructions. Shown are: TBP (red), TFIIA (orange), TFIIB (blue), DNA (green, yellow and cyan), Pol II (grey), TFIIF (purple), and TFIIE (maroon). From: He Y., Fang J., Taatjes, D. J., and Nogales, E. 2013. Nature.|
According to Nogales, this is only the beginning. “If we want to get at what is different between one gene and another, we have to start building up even larger complexes,” she says. The next step is to add in the factors that allow the transcription machinery to recognize genes in a regulated fashion.