Assembly of RNA polymerase II transcription initiation complexes.

The first step of eukaryotic gene expression is the assembly of RNA polymerase II and general transcription factors on promoter DNA. This highly regulated process involves ∼80 different proteins that together form the preinitiation complex (PIC). Decades of work have gone into understanding PIC assembly using biochemical and structural approaches. These efforts have yielded significant but partial descriptions of PIC assembly. Over the past few years, cryo-electron microscopy has provided the first high-resolution structures of the near-complete mammalian PIC assembly. These structures have revealed that PIC assembly is a highly dynamic process. This review will summarize recent structural findings and discuss their implications for understanding cell type-specific gene expression.