Ribosomes are macromolecular machines that are responsible for production of every protein in a living cell. Yet we do not know the details about how these machines are formed. The ribosome consists of four RNA strands and roughly 80 proteins that associate with each other in the nucleolus and form pre-ribosomal complexes. Eukaryotes, in contrast to prokaryotes, need more than 200 non-ribosomal factors to assemble ribosomes. These associate with pre-ribosomal complexes at different stages as they travel from the nucleolus to the cytoplasm and are required for pre-rRNA processing. We do however lack knowledge about the molecular function of most of these factors and what enables pre-rRNA processing. Especially, information is missing about how non-ribosomal factors influence folding of the pre-rRNA and to what extent the pre-ribosomal complexes are restructured during their maturation. 

This thesis aims to obtain a better understanding of the earliest events of ribosome assembly, namely those that take place in the nucleolus. This has been achieved by studying the essential protein Mrd1 by mutational analysis in the yeast Saccharomyces cerevisiae as well as by obtaining structural information of nucleolar pre-ribosomal complexes. Mrd1 has a modular structure consisting of multiple RNA binding domains (RBDs) that we find is conserved throughout eukarya. We show that an evolutionary conserved linker region of Mrd1 is crucial for function of the protein and likely forms an essential module together with adjacent RBDs. By obtaining structural information of pre-ribosomal complexes at different stages, we elucidate what structuring events occur in the nucleolus.  We uncover a direct role of Mrd1 in structuring the pre-rRNA in early pre-ribosomal complexes, which provides an explanation for why pre-rRNA cannot be processed in Mrd1 mutants.