Ribosomal ribonucleic acid (rRNA) is the RNA component of the ribosome, and is essential for protein synthesis in all living organisms. It constitutes the predominant material within the ribosome, which is approximately 60% rRNA and 40% protein by weight, or 3/5 of ribosome mass. Ribosomes contain two major rRNAs and 50 or more proteins. The ribosomal RNAs form two subunits, the large subunit (LSU) and small subunit (SSU). The LSU rRNA acts as a ribozyme, catalyzing peptide bond formation. rRNA sequences are widely used for working out evolutionary relationships among organisms, since they are of ancient origin and are found in all known forms of life.
The ribosomal RNAs complex with proteins to form two subunits, the large subunit (LSU) and small subunit (SSU). During translation, mRNA is sandwiched between the small and large subunits, and the ribosome catalyzes the formation of a peptide bond between the two amino acids that are contained in the rRNA.
A ribosome also has three binding sites called A, P, and E.
A single mRNA can be translated simultaneously by multiple ribosomes.
Both prokaryotic and eukaryotic ribosomes can be broken down into two subunits (the S in 16S represents Svedberg units), nt= length in nucleotides of the respective rRNAs, for exemplary species Escherichia coli (prokaryote) and human (eukaryote):
|Type||Size||Large subunit (LSU rRNA)||Small subunit (SSU rRNA)|
|prokaryotic||70S||50S (5S : 120 nt, 23S : 2906 nt)||30S (16S : 1542 nt)|
|eukaryotic||80S||60S (5S : 121 nt, 5.8S : 156 nt, 28S : 5070 nt)||40S (18S : 1869 nt)|
Note that the S units of the subunits (or the rRNAs) 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 nt units can be added as these represent the integer number of units in the linear rRNA polymers (for example, the total length of the human rRNA = 7216 nt).
The large 50S ribosomal subunit contains two rRNA species (the 5S and 23S ribosomal RNAs).
Bacterial 16S ribosomal RNA, 23S ribosomal RNA, and 5S rRNA genes are typically organized as a co-transcribed operon.
Archaea contains either a single rDNA operon or multiple copies of the operon.
The 3' end of the 16S ribosomal RNA (in a ribosome) binds to a sequence on the 5' end of mRNA called the Shine-Dalgarno sequence.
In contrast, eukaryotes generally have many copies of the rRNA genes organized in tandem repeats; in humans approximately 300–400 repeats are present in five clusters (on chromosomes 13, 14, 15, 21 and 22). Sequence variation in rRNA has been observed both within and between human individuals and certain variants are expressed in a tissue-specific manner in mice. Because of their special structure and transcription behaviour, rRNA gene clusters are commonly called "ribosomal DNA" (note that the term seems to imply that ribosomes contain DNA, which is not the case).
Mammalian cells have 2 mitochondrial (12S and 16S) rRNA molecules and 4 types of cytoplasmic rRNA (the 28S, 5.8S, 18S, and 5S subunits). The 28S, 5.8S, and 18S rRNAs are encoded by a single transcription unit (45S) separated by 2 internally transcribed spacers. The 45S rDNA is 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.
In prokaryotic cells, each rRNA gene or operon is transcribed into a single RNA precursor that includes 16S, 23S, 5S rRNA and tRNA sequences along with transcribed spacers. The RNA processing then begins before the transcription is complete. During processing reactions, the rRNAs and tRNAs are released as separate molecules.
The genes coding for 18S, 28S and 5.8S rRNA, located in the nucleolus organizer region, are transcribed into a large pre-rRNA molecule by RNA polymerase I. Each large pre-rRNA contains 18S, 28S and 5.8S sequences which are separated by external and internal transcribed spacer sequences. During processing reactions, the 18S, 28S and 5.8S rRNA are released as individual molecules. Processing reactions involve exo- and endo-nucleolytic cleavages guided by snoRNA (small nucleolar RNAs) in complex with proteins. The genes for 5S rRNA are located outside the nucleolus and are transcribed into pre-5S rRNA by RNA polymerase III. The pre-5S rRNA enters the nucleolus for processing and assembly with 28S and 5.8S rRNA to form the large subunit. 18S rRNA forms the small ribosomal subunits by combining with ribosomal proteins.
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 Shine–Dalgarno sequence upstream of the start codon in mRNA.
These denote genes encoding for the proteins of the ribosome and are transcribed as mRNA, not rRNA.