Ribosomal RNAs (rRNAs) constitute ribosomes where protein synthesis occurs. rRNA and ribosomal proteins combine to form ribosomes. Both prokaryotic and eukaryotic rRNA are made from longer precursors called preribosomal RNAs, or pre-rRNA .
In bacteria, a single 30S, 6500 nucleotides long, RNA precursor, after processing, makes 16S, 23S and 5S rRNAs. RNA at both ends of 30S precursor and between the rRNAs is removed during processing. The E. coli genome encodes 7 pre-rRNA molecules. All these genes have identical rRNA coding regions; but the regions in between the coding regions differ. The regions between the genes for 16S and 23S rRNA code for one or two tRNAs.
In eukaryotes, a 45S pre-rRNA transcript is processed to give rise to 18S, 28S and 5.8S rRNAs. The processing takes place in the nucleolus. The 5S rRNA is synthesized as a separate transcript.