We have isolated a large protein complex from the yeast nucleolus (SNOS for subnucelolar structure). Among the identified proteins were RNA polymerase I (pol I), pol I transcription factors TBP, Rrn3p, Rrn10p and Reb1p and rRNA processing factors Nop1p, Cbf5p, Nhp2p and Rrp5p. The complex supports accurate transcription, termination and modification of rRNA suggesting the existence of a cooperative network to produce mature rRNA. Since most of the compontents (?30) within the large isolated complex remain to be determined, MALDI mass spectrometry should serve as a tool to identify other (novel) nucleolar proteins. Among the components that we have already identified within the subnucleolar structure are two novel proteins which are members of a eukaryotic protein family based on their striking conservation of certain subdomains. Since one of the conserved domain predicts selective binding to DNA the interaction of the proteins with DNA, especially with domains of the rDNA unit will be analysed. Two further identified factors ot the subnucleolar structure are obviously involved in transcribing the rDNA unit. They were both originally described to be required to transcribe efficiently mRNA. We will use a combination of biochemical, genetic and cell biological methods to analyse the roles of the identified (novel) proteins in synthesis and processing of rRNA and their possible involvement in building up the nucleolar structure.
