Ribosomal RNA Gene Structure and Processing

The genes for rRNA, like those for tRNA, can be present in multiple copies, and the numbers vary among species (Table 14.4); all copies of the rRNA gene in a species are identical or nearly identical. In bacteria, rRNA genes are dispersed, but, in eukaryotic cells, they are clustered, with the genes arrayed in tandem, one after another.

Eukaryotic cells possess two types of rRNA genes: a large one that encodes 18S rRNA, 28S rRNA, and 5.8S rRNA, and a small one that encodes the 5S rRNA. All three bacterial rRNAs (23S rRNA, 16S rRNA, and 5S rRNA) are encoded by a single type of gene.

Ribosomal RNA is processed in both bacterial and eukaryotic cells. In E. coli, the immediate product of transcription is a 30S rRNA precursor (I Figure 14.24a). Methyl groups (CH3) are added to specific bases and to the 2' carbon of some of the ribose sugars of this 30S precursor, which is then cleaved into several pieces and trimmed to produce 16S rRNA, 23S rRNA, and 5S rRNA, along with one or more tR-NAs. All rRNA genes in E. coli produce the same three rRNA molecules, but the number and location of these rRNAs within the 30S rRNA transcript differ among genes.

Eukaryotic rRNAs undergo similar processing (iFigure 14.24b). Small nucleolar RNAs help to cleave and


Escherichia coli




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