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How are pre-rRNA transcripts processed in bacteria?A.
In bacteria, 16S, 23S, and 5S rRNAs (and some tRNAs, although most tRNAs are encoded elsewhere) arise from a single 30S RNA precursor of about 6,500 nucleotides. RNA at both ends of the 30S precursor and segments between the rRNAs are removed during processing. The 16S and 23S rRNAs contain modified nucleosides.
B.
Before cleavage, the 30S RNA precursor is methylated at specific bases, and some uridine residues are converted to pseudouridine or dihydrouridine residues. The methylation reactions are of multiple types, some occurring on bases and some on 2′-hydroxyl groups.
C.
Cleavage liberates precursors of rRNAs and tRNA(s). Cleavage at different sites is carried out by the enzymes RNase III, RNase P, and RNase E. RNase P is a ribozyme.
D.
The final 16S, 23S, and 5S rRNA products result from the action of a variety of specific nucleases. The seven copies of pre-rRNA gene in the
E.
coli chromosome differ in the number, location, and identity of tRNAs included in the primary transcript. Some copies of the gene have additional tRNA gene segments between the 16S and 23S rRNA segments and at the far 3′ end of the primary transcript.
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