Association of yeast RNA polymerase I with a nucleolar substructure activein rRNA synthesis and processing

A novel ribonucleoprotein complex enriched in nucleolar proteins was purifi ed from yeast extracts and constituents were identified by mass spectrometr y. When isolated from rapidly growing cells, the assembly contained ribonuc leic acid (RNA) polymerase (pol) I, and some of its transcription factors l ike TATA-binding protein (TBP), Rrn3p, Rrn5p, Rm7p, and Reb1p along with rR NA processing factors, like Nop1p, Cbf5p, Nhp2p, and Rrp5p. The small nucle olar RNAs (snoRNAs) U3, U14, and MRP were also found to be associated with the complex, which supports accurate transcription, termination, and pseudo uridylation of rRNA. Formation of the complex did not depend on pol I, and the complex could efficiently recruit exogenous pol I into active ribosomal DNA (rDNA) transcription units. Visualization of the complex by electron m icroscopy and immunogold labeling revealed a characteristic cluster-forming network of nonuniform size containing nucleolar proteins like Nop1p and Fp r3p and attached pol I. Our results support the idea that a functional nucl eolar subdomain formed independently of the state of rDNA transcription may serve as a scaffold for coordinated rRNA synthesis and processing.