Description:

The Department of Biochemistry and Molecular Genetics Departmental Seminar Series presents:

Yuan He, PhD
Assistant Professor, Department of Molecular Biosciences
Northwestern University Feinberg School of Medicine

Accurately regulated eukaryotic gene transcription requires the coordinated activity of a large number of proteins. Among these, the basal transcriptional machinery includes RNA polymerase II (pol II) along with a series of general transcription factors (TFIIA, IIB, IID, IIE, IIF, and IIH), all of which assemble onto the core promoter DNA in a stepwise manner. This preinitiation complex (PIC) is essential for correct initiation, promoter melting and escape. In spite of its huge biological significance, the molecular assembly details of this enormous complex still remain elusive, due to the complexity of the system and the consequently limited structural information available. In order to fully characterize the sequential molecular assembly in human transcription initiation, we have developed an in vitro system for reconstitution and purification of a simplified PIC, in which TBP substitutes for IID, and that ultimately contains 31 polypeptides. We have used this system for the stepwise visualization of key PIC intermediates of increasing size using cryo-electron microscopy (cryo-EM). Our structural analyses provide pseudo-atomic models at various stages of transcription initiation that suggest a critical role for TFIIF in engaging Pol II on promoter DNA, start site selection and stabilization of the transcription bubble. Comparison of the open versus closed pre-initiation complex and localization of the TFIIH helicases XPD and XPB relative to the rest of the complex support that a DNA translocation activity of XPB plays an essential role in promoter opening.