Ribosomal RNA Genes in Bacteria

 
Ribosomal RNA is the major component of ribosomes [The Compositon of Ribosomes]. This RNA, by itself, is capable of catalyzing the amino acid joining reaction (peptidyl transferase) during translation but inside the cell the RNA is closely associated with many proteins to form the complete ribosome.



In bacteria (prokaryotes) there are three different ribosomal RNAs called 5S, 16S, and 23S. Eukaryotes have homologous RNAs called 5S, 28S, and 18S ribosomal RNAs. In addition, they have a 5.8S RNA that is homologous to one end of the prokaryotic 23S RNA. (The sizes of eukaryotic RNAs vary considerably and different species might have slightly different names.)

The 5S, 16S and 23S ribosomal RNAs are produced from a single operon in bacteria. An operon is a group of genes that are cotranscribed; that is, they form part of a single transcriptional unit. In this case the parts of the genome that specify each of the ribosomal RNAs are contiguous and the "genes" are transcribed into a single precursor RNA beginning at the promoter (P) and ending at the terminator (t). This large precursor RNA is then cleaved to make the smaller mature ribosomal RNAs. This is one of the problem cases when one is attempting to define a gene [What Is a Gene?].



Bacterial cells need a lot of ribosomal RNA so they usually have multiple copies of the ribosomal RNA operon (rrn). In E. coli for example, there are usually seven different rrn operons called rrnA, rrnB, etc.

In bacterial genomes the operons are located in different parts of the genome—they are unlinked. (By contrast, in eukaryotic genomes they cluster in large tandem arrays.) The example shown above is the rrnC operon in E. coli. The rectangular boxes represent regions that are transcribed into the large RNA precursor. Open boxes are pieces at the ends that are removed by direct cleavage. The purple boxes are internal regions that removed by making two cuts, one on either side.

In this operon there are four transfer RNA genes (tRNA) embedded in the transcriptional unit. These are processed as described in Transfer RNA: Synthesis. The parts of the transcription unit located between the "genes" (i.e., the purple regions) are called "transcribed spacers." When processing is complete the mature 5S, 16S and 23S RNAs are ready to be incorporated into ribosome and the four tRNAs are ready to act in translation.

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