Speaker: Benoit Coulombe A panorama of the protein complexes formed by the RNA polymerase II transcription machinery in mammals We have performed a survey of protein complexes containing components of the transcription and RNA processing machineries in the soluble compartment of mammalian cells using protein affinity purification coupled to mass spectrometry (MS). High confidence interactions were selected computationally by applying specific filters and integrating the results of multiple purifications. Thirty tagged polypeptides yielded a network of 489 interactions. Remarkably, the network is significantly enriched in proteins known to regulate the folding and assembly of protein complexes, uncovering a novel regulatory mechanism that targets transcription and RNA processing factors prior to their recruitment to active genomic loci. The network also contains a number of previously-uncharacterized proteins to which we have inferred putative functions, including a 7SK snRNA-specific methyltransferase that is part of a complex containing RNA processing and transcription factors, including elongation factor P-TEFb. Our results define the first high-density interaction network involving the cellular machinery that interprets mammalian genomes.