Ribosomal RNA (rRNA) is the central component of the ribosome, the protein manufacturing machinery of all living cells. The function of the rRNA is to provide a mechanism for decoding mRNA into amino acids and to interact with the tRNAs during translation by providing peptidyl transferase activity.
Inside the ribosome
The ribosome is composed of two subunits, named for how rapidly they sediment when subject to centrifugation. tRNA is sandwiched between the small and large subunits and the ribosome catalyzes the formation of a peptide bond between the 2 amino acids that are contained in the tRNA.
The ribosome also has 3 binding sites called A, P, and E.
- The A site in the ribosome binds to an aminoacyl-tRNA (a tRNA bound to an amino acid).
- The NH2 group of the aminoacyl-tRNA which contains the new amino acid, attacks the carboxyl group of peptidyl-tRNA (contained within the P site) which contains the last amino acid of the growing chain called peptidyl transferase reaction.
- The tRNA that was holding on the last amino acid is moved to the E site, and what used to be the aminoacyl-tRNA is now the peptidyl-tRNA.
A single mRNA can be translated simultaneously by multiple ribosomes.
Prokaryotes vs. Eukaryotes
|Type||Size||Large subunit||Small subunit|
|prokaryotic||70S||50S (5S, 23S)||30S (16S)|
|eukaryotic||80S||60S (5S, 5.8S, 28S)||40S (18S)|
Note that the S units of the subunits cannot simply be added because they represent measures of sedimentation rate rather than of mass. The sedimentation rate of each subunit is affected by its shape, as well as by its mass.
The large 50S ribosomal subunit contains two rRNA species (the 5S and 23S rRNAs).
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).
Mammalian cells have 2 mitochondrial (12S and 16S) rRNA molecules and 4 types of cytoplasmic rRNA (28S, 5.8S, 5S (large ribosome subunit) and 18S (small subunit)). 28S, 5.8S, and 18S rRNAs are encoded by a single transcription unit (45S) separated by 2 Internally transcribed spacer (ITS). The 45S rDNA organized into 5 clusters (each has 30-40 repeats) on chromosomes 13, 14, 15, 21, and 22. These are transcribed by RNA polymerase I. 5S occurs in tandem arrays (~200-300 true 5S genes and many dispersed pseudogenes), the largest one on the chromosome 1q41-42. 5S rRNA is transcribed by RNA polymerase III.
The tertiary structure of the small subunit ribosomal RNA (SSU rRNA) has been resolved by X-ray crystallography . The secondary structure of SSU rRNA contains 4 distinct domains -- the 5', central, 3' major and 3' minor domains. A model of the secondary structure for the 5' domain (500-800 nucleotides) is shown.
Translation is the net effect of proteins being synthesized by ribosomes, from a copy (mRNA) of the DNA template in the nucleus. One of the components of the ribosome (16s rRNA) base pairs complementary to a sequence upstream of the start codon in mRNA.
Importance of rRNA
- 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. For this reason many thousands of rRNA sequences are known and stored in specialized databases such as RDP-II and the European SSU database.
- Yusupov MM, Yusupova GZ, Baucom A; et al. (2001). "Crystal structure of the ribosome at 5.5 A resolution". Science. 292 (5518): 883–96. PMID 11283358. doi:10.1126/science.1060089.
- Cole, JR (2003). "The Ribosomal Database Project (RDP-II): previewing a new autoaligner that allows regular updates and the new prokaryotic taxonomy". Nucleic Acids Res. 31: 442–443. PMID 12520046.
- Wuyts, J (2002). "The European database on small subunit ribosomal RNA". Nucleic Acids Res. 30: 183–185. PMID 11752288.
- Ribosomal RNA by Denis LJ Lafontaine and David Tollervey
- European database on small subunit ribosomal RNA
- SILVA Database Project (also includes Eukaryotes and 23S / 28S)
- Ribosomal Database Project II
- 16S rRNA, BioMineWiki
- Rfam page for Small subunit ribosomal RNA, 5' domain
ar:رنا الريبوسومي br:Trenkenn ribonukleek ribosom ca:ARN ribosòmic cs:RRNA da:RRNA de:Ribosomale RNA el:Ριβοσωμικό RNA ko:RRNA it:RNA ribosomiale he:RRNA la:RRNA nl:Ribosomaal RNA sk:Ribozomálna ribonukleová kyselina sv:Ribosom-RNA