Fig. 4.

Processing of rRNA operons in E. coli. The rrnB and rrnC operons are shown as model operons. RNase III (RIII) cleaves the 30S rRNA transcript first within the double-stranded stems formed by the spacer sequences adjacent to the mature 16S and 23S rRNAs, generating 17S, 25S, and 9S pre-rRNAs. The functional mature 16S rRNA is generated from 17S pre-rRNA after initial RNase E (E) cleavage followed by RNase G (G) at the mature 5′ end and removal of extra 33 nts at the 3′ ends by YbeY (Y) along with multiple exoribonucleases (not shown). A p5S precursor is generated from the the 9S precursor by initial RNase E cleavage at three nt upstream (E) and downstream (E) of the mature termini of the mature 5S rRNA. The mature 5′ end of the tRNAs are generated by RNase P (P) cleavage. Exoribonucleases (X) (primarily RNase T) are responsible for the 3′ end maturation of the tRNAs, 23S rRNA, and 5S rRNA, but the ribonuclease(s) (?) responsible for the maturation of 5′ ends of 23S and 5S rRNAs remain unidentified. The model is not drawn to scale.