Post-transcriptional Control of Gene Expression
Ribosomal RNAs (rRNAs), components of ribosomes, the large ribonucleoprotein complexes (RNPs) that translate mRNA into protein in the cytoplasm of eukaryotic cells, are transcribed as a pre-rRNA which is modified, cleaved and assembled with ribosomal proteins to make functional ribosomes. The efficient processing of rRNAs is therefore an essential step for achieving successful translation. This process involves a large number of factors, estimated to be approximately 150 in the yeast Saccharomyces cerevisiae, that are conserved throughout evolution. By exploring the biochemical and molecular biology possibilities of yeast as a model system we can greatly improve our understanding of the synthesis and regulation of the eukaryotic translational machinery. Proteomics and functional analyses have been used in our laboratory to identify and characterize novel factors involved in pre-rRNA processing, which participate in different steps of the pathway. A major player in this process is the exosome complex acting in RNA processing and turnover. We have been able to reconstitute the exosome activity in vitro both from yeast and from archaea, and to identify proteins interacting with the exosome that affect its activity. The exosome core subunit Rrp43 interacts with the nucleolar proteins Nop17 and Nip7. Nop17 is required for the early steps of rRNA maturation, regulating nucleotide methylation. Nip7 is required for proper processing of the 27S pre-rRNA and in the later steps of large ribosomal subunit assembly. Nop17 and Nip7 also interact with Nop53, which binds directly to 27S pre-rRNA, thereby regulating the function of the exosome. Nop17 also interacts with the essential splicing factor Cwc24 that binds the intron 5’-end, important for the first splicing reaction. By understanding the molecular function of these factors, and the functional interactions between them, we can start to define a model for the regulation of the whole process of ribosome biosynthesis and function, not only in yeast, but also in other eukatyotes. The information obtained from the yeast system can be useful for future diagnostics and treatment of human diseases, such as ribosomopathies.