Probing ribosomal RNA structural rearrangements: a time lapse of ribosome assembly dynamics
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Ribosome synthesis is a very complex and energy consuming process in which pre-ribosomal RNA (pre-rRNA) processing and folding events, sequential binding of ribosomal proteins and the input of approximately 200 trans-acting ribosome assembly factors need to be tightly coordinated. In the yeast Saccharomyces cerevisiae, ribosome assembly starts in the nucleolus with the formation of a very large 90S-sized complex. This ~2.2MDa pre-ribosomal complex is subsequently processed into the 40S and 60S assembly intermediates (pre-40S and pre-60S), which subsequently mature largely independently. Although we have a fairly complete picture of the protein composition of these pre-ribosomes, still very little is known about the rRNA structural rearrangements that take place during the assembly of the 40S and 60S subunits and the role of the ribosome assembly factors in this process. To address this, the Granneman lab developed a method called ChemModSeq, which made it possible to generate nucleotide resolution maps of RNA flexibility in ribonucleoprotein complexes by combining SHAPE chemical probing, high-throughput sequencing and statistical modelling. By applying ChemModSeq to ribosome assembly intermediates, we were able to obtain nucleotide resolution insights into rRNA structural rearrangements during late (cytoplasmic) stages of 40S assembly and for the early (nucleolar) stages of 60S assembly. The results revealed structurally distinct cytoplasmic pre-40S particles in which rRNA restructuring events coincide with the hierarchical dissociation of assembly factors. These rearrangements are required to trigger stable incorporation of a number of ribosomal proteins and the completion of the head domain. Rps17, one of the ribosomal proteins that fully assembled into pre-40S complexes only at a later assembly stage, was further characterized. Surprisingly, my ChemModSeq analyses of nucleolar pre-60S complexes indicated that most of the rRNA folding steps take place at a very specific stage of maturation. One of the most striking observations was the stabilization of 5.8S pre-rRNA region, which coincided with the dissociation of the assembly factor Rrp5 and stable incorporation of a number of ribosomal proteins.