"Through cryo-EM and extensive image-sorting, we found that TFIID exhibits a surprising degree of flexibility, moving its lobe A, a region that covers approximately one-third of the complex, by 100 angstroms across its central channel," says Cianfrocco, lead author of the Cell paper. "This movement of the lobe A is absolutely essential for TFIID to bind to DNA."
Nogales says that while many macromolecular complexes are known to be flexible, this typically involves the limited movement of a small region within the complex, or some tiny motion of the entire complex. The movement of TFIID's lobe A represents an entire restructuring that dramatically alters what the molecule can do. In the canonical state, TFIID's lobe A is bound to its lobe C, which appears to be the preferred form of free TFIID. In the rearranged state, TFIID's lobe A is bound to its lobe B, which is the state in which it can then strongly bind to DNA promoters.
"The TFIIA molecule serves as the mediator for this transition, maintaining TFIID in the canonical state in the absence of DNA and initiating the formation of the rearranged state in the presence of promoter DNA," Cianfrocco says. "Without the presence of TFIIA, the binding of TFIID to DNA is very weak."
Nogales and her colleagues are now studying how TFIID, once it is bound to DNA, recruits the rest of the machinery required to transcribe the genetic message into RNA.
"Our new work will involve constructing a macromolecular complex that is well over two
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory