This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. RNA polymerase II (pol II), the central enzyme of gene expression, synthesizes all mRNA in eukaryotes. The intricate regulation of pol II transcription underlies cell growth and differentiation. Pol II is both a machine, capable of DNA unwinding, RNA polymerization, and proofreading, and also a platform for the assembly of even larger initiation and elongation complexes. The recent structure determination of ten-subunit, 0.5 mDal, Saccharomyces cerevisiae (pol II) at 2.8 ¿ resolution, and of a transcribing pol II complex at 3.3 ¿ resolution, opens the way to many lines of investigation. Structures of cocrystals of pol II with interacting molecules can be solved, the full power of site-directed mutagenesis can be brought to bear on understanding the transcription mechanism, and so forth. Structure determination of the pol II transcription machinery is essential for understanding the mechanism and regulation of transcription.