出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2016/09/18 09:28:26」(JST)
リボソームRNAはリボソームを構成するRNAであり、RNAとしては生体内でもっとも大量に存在する(7~8割くらい)。通常rRNAと省略して表記される。
原核生物では沈降係数に由来する命名で23Sと5Sがリボソーム大サブユニット(50Sサブユニット)に含まれる。また小サブユニット(30Sサブユニット)には16SrRNAが含まれる。クレンアーキオータ(5Sが独立している)を除き16S, 23S, 5Sの順に並んだオペロン構造を持っている。
真核生物の大サブユニット(60Sサブユニット)には一般に28Sと5.8S、5S rRNA、小サブユニット(40Sサブユニット)には18S rRNAが含まれるが、種によってその数字には若干の違いがある。 ヒトにおいてはこのうち28S、5.8S、18S RNAは一つの転写単位に由来する。これはrRNA前駆体と呼ばれる約2 kbのRNAであり、RNAポリメラーゼIによって核小体で転写される。転写されたrRNA前駆体は、snoRNAなどの様々なRNAやタンパク質の働きをうけて、不要な部分が取り除かれ、また修飾を受けてrRNAになる。一方、5S RNAはRNAポリメラーゼIIIにより転写される。 rRNAはタンパク質合成の触媒反応の活性中心を形成していると考えられている。
リボソーム粒子 | 粒子構成 | rRNA | タンパク質 |
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70Sリボソーム(原核生物) | 50Sサブユニット | 5S, 23S | 34種 |
30Sサブユニット | 16S | 21株 | |
70Sリボソーム(古細菌)[2] | 50Sサブユニット | 5S, 23S | 40種 |
30Sサブユニット | 16S | 28株 | |
80Sリボソーム(真核生物) | 60Sサブユニット | 28S, 5.8S, 5S | 46種 |
40Sサブユニット | 18S | 32種 |
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In molecular biology, 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. 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 (rRNAs) | Small subunit (rRNA) |
prokaryotic | 70S | 50S (5S : 120 nt, 23S : 2906 nt) | 30S (16S : 1542 nt) |
eukaryotic | 80S | 60S (5S : 121 nt,[1] 5.8S : 156 nt,[2] 28S : 5070 nt[3]) | 40S (18S : 1869 nt[4]) |
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).
In prokaryotes a small 30S ribosomal subunit contains the 16S ribosomal RNA.
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.
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.
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). 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).
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 28S in mammals, 25S in plants, rRNAs).
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.[5] 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 Shine–Dalgarno sequence upstream of the start codon in mRNA.
Ribosomal RNA characteristics are important in evolution, thus taxonomy, and medicine.
These denote genes encoding for the proteins of the ribosome and are transcribed as mRNA, not rRNA.
Types of nucleic acids
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Constituents |
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Ribonucleic acids (coding, non-coding) |
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Deoxyribonucleic acids |
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Analogues |
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Cloning vectors |
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Ribosomal RNA / ribosome subunits
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Prokaryotes (70S) |
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Eukaryotes (80S) |
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リンク元 | 「リボソーム」「リボソームRNA」「ribosomal RNA」 |
拡張検索 | 「rRNAプロモーター」「rRNA promoter」「rRNA gene」「rRNA operon」 |
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