Ribosomal RNAs are tolerant toward genetic insertions: evolutionary origin of the expansion segments

Ribosomal RNAs (rRNAs), assisted by ribosomal proteins, form the basic structure of the ribosome, and play critical roles in protein synthesis. Compared to prokaryotic ribosomes, eukaryotic ribosomes contain elongated rRNAs with several expansion segments and larger numbers of ribosomal proteins. To investigate architectural evolution and functional capability of rRNAs, we employed a Tn5 transposon system to develop a systematic genetic insertion of an RNA segment 31 nt in length into Escherichia coli rRNAs. From the plasmid library harboring a single rRNA operon containing random insertions, we isolated surviving clones bearing rRNAs with functional insertions that enabled rescue of the E. coli strain (Δ7rrn) in which all chromosomal rRNA operons were depleted. We identified 51 sites with functional insertions, 16 sites in 16S rRNA and 35 sites in 23S rRNA, revealing the architecture of E. coli rRNAs to be substantially flexible. Most of the insertion sites show clear tendency to coincide with the regions of the expansion segments found in eukaryotic rRNAs, implying that eukaryotic rRNAs evolved from prokaryotic rRNAs suffering genetic insertions and selections.