Importance of Transient Structures During Post-transcriptional Refolding of the Pre-23 S rRNA and Ribosomal Large Subunit Assembly

An important step of ribosome assembly is the folding of the rRNA into a functional structure. Despite knowledge of the folded state of rRNA in the ribosomal subunits, there is very little information on the rRNA folding pathway. We are interested in understanding how the functional structure of rRNA is formed and whether the rRNA folding intermediates have a role in ribosome assembly. To this end, transient secondary structures around both ends of pre-23 S rRNA were analyzed by a chemical probing approach, using pre-23 S rRNA transcripts. Metastable hairpin loop structures were found at both ends of 23 S rRNA. The functional importance of the transient structures around the ends of 23 S rRNA was tested by mutations that alter only the transient structure. The effect of mutations on 23 S rRNA folding was tested in vitro and in vivo. It was found that both stabilization and destabilization of the transient structure around the 5′ end of 23 S rRNA inhibits post-transcriptional refolding in vitro and ribosome formation in vivo. The data suggest that the transient structure of rRNA has a function during 23 S rRNA folding and thereby in ribosome assembly.