Multiple exoribonucleases catalyze maturation of the 3′ terminus of 16S ribosomal RNA (rRNA)

Processing of ribosomal RNA (rRNA) precursors is an important component of RNA metabolism in all cells. However, in no system have we yet identified all the RNases involved in this process. Here, we show that four 3′→5′-exoribonucleases, RNases II, R, and PH, and polynucleotide phosphorylase (PNPase), participate in maturation of the 3′ end of 16S rRNA. In their absence, 16S precursor molecules with 33 extra 3′-nt accumulate; however, the presence of any one of the four RNases is sufficient to allow processing to occur, although with different efficiencies. Additionally, we find that in the absence of 3′ maturation, 5′ processing proceeds much less efficiently. Moreover, mutant 30S particles, containing immature 16S rRNA, form 70S ribosomes very poorly. These findings, together with the earlier discovery that RNases E and G are the 5′-processing enzymes, completes the catalogue of RNases involved in maturation of Escherichia coli 16S rRNA.