Ribosomal RNA

} Ribosomal RNA (rRNA) is the central component of the ribosome, the protein manufacturing machinery of all living cells. The rRNA is synthesized in the nucleolus. These machines then self-assemble into two complex folded structures (the large and the small subunits) in the presence of 70 – 80 ribosomal proteins.

The function of the rRNA is to provide a mechanism for decoding mRNA into amino acids (at center of small ribosomal subunit) and to interact with the tRNAs during translation by providing peptidyl transferase activity (large subunit). Accuracy of translation is provided by both subunits.

Ribosomal RNA characteristics are important in medicine and in evolution.


 * rRNA is the target of several clinically relevant antibiotics: Chloramphenicol, Erythromycin, Kasugamycin, Micrococcin, Paromomycin, Ricin, Sarcin, Spectinomycin, Streptomycin, and Thiostrepton.


 * rRNA is the most conserved (least variable) gene in all cells. For this reason, genes that encode the rRNA (rDNA) are sequenced to identify an organism's taxonomic group, calculate related groups, and estimate rates of species divergence.

In Bacteria, Archaea, Mitochondria, and Chloroplasts a small ribosomal subunit contains the 16S rRNA, where the S in 16S represents Svedberg units; the large ribosomal subunit contains two rRNA species (the 5S and 23S rRNAs). Bacterial 16S, 23S, and 5S rRNA genes are typically organized as a co-transcribed operon. There may be one or more copies of the operon dispersed in the genome (for example, Escherichia coli has seven). Archaea contains either a single rDNA operon or multiple copies of the operon.

In contrast, Eukaryotes generally have many copies of the rRNA genes organized in tandem repeats; in humans approximately 300–400 rDNA repeats are present in five clusters (on chromosomes 13, 14, 15, 21 and 22). The 18S rRNA in most eukaryotes is in the small ribosomal subunit, and the large subunit contains three rRNA species (the 5S, 5.8S and 25S/28S rRNAs)

Nucleolar dominance has also been shown for rRNA genes. In some organisms, particularly plants, when two nuclei are combined into a single cell during hybridization the developing organism can 'choose' one set of rRNA genes for transcription. The rRNA genes of the other parent are suppressed and not generally transcribed, though reactivation of the suppressed or 'inferior' rRNA genes may occasionally occur. This selective preference of transcription of rRNA genes is termed nucleolar dominance.