Gateway Role for rRNA Precursors in Ribosome Assembly

In Escherichia coli, rRNAs are initially transcribed with precursor sequences, which are subsequently removed through process-ing reactions. To investigate the role of precursor sequences, we analyzed ribosome assembly in strains containing mutations in the processing RNases. We observed that defects in 23S rRNA processing resulted in an accumulation of ribosomal subunits and caused a significant delay in ribosome assembly. These observations suggest that precursor residues in 23S rRNA control ribo-some assembly and could be serving a regulatory role to couple ribosome assembly to rRNA processing. The possible mecha-nisms through which rRNA processing and ribosome assembly could be linked are discussed. The assembly of ribosomes from their individual RNA and pro-tein components is an elaborate process that involves dozens of macromolecular components. Despite its complexity, this pro-cess proceeds rapidly and is normally completed within minutes in many prokaryotic organisms (7, 12, 19). During the assembly process, rRNAs also undergo processing, with the result that the newly assembled ribosomes generally contain rRNAs withmature 3 = and 5 = ends. Ribosome assembly and rRNA processing have been exten-sively studied in Escherichia coli, an organism that has served as