Editing Ribosome (section)

=== Prokaryotic ribosomes ===
Prokaryotic ribosomes are around 20&nbsp;[[Nanometre|nm]] (200&nbsp;[[Ångström|Å]]) in diameter and are composed of 65% rRNA and 35% [[ribosomal protein]]s.<ref>{{cite journal|author1-link=Charles Kurland| vauthors = Kurland CG |title=Molecular characterization of ribonucleic acid from Escherichia coli ribosomes|journal=Journal of Molecular Biology|volume=2|issue=2|pages=83–91|doi=10.1016/s0022-2836(60)80029-0|year=1960}}</ref> Eukaryotic ribosomes are between 25 and 30 [[Nanometre|nm]] (250–300 Å) in diameter with an rRNA-to-protein ratio that is close to 1.<ref>{{cite journal | vauthors = Wilson DN, Doudna Cate JH | title = The structure and function of the eukaryotic ribosome | journal = Cold Spring Harbor Perspectives in Biology | volume = 4 | issue = 5 | pages = a011536 | date = May 2012 | pmid = 22550233 | pmc = 3331703 | doi = 10.1101/cshperspect.a011536 }}</ref> [[Crystallography|Crystallographic]] work<ref>{{cite journal | vauthors = Nissen P, Hansen J, Ban N, Moore PB, Steitz TA | title = The structural basis of ribosome activity in peptide bond synthesis | journal = Science | volume = 289 | issue = 5481 | pages = 920–30 | date = August 2000 | pmid = 10937990 | doi = 10.1126/science.289.5481.920 | bibcode = 2000Sci...289..920N | s2cid = 8370119 | url = http://pdfs.semanticscholar.org/0baa/9ba2a4d098674984113142bbf1ffca35700c.pdf | archive-url = https://web.archive.org/web/20201130102727/http://pdfs.semanticscholar.org/0baa/9ba2a4d098674984113142bbf1ffca35700c.pdf | url-status = dead | archive-date = 2020-11-30 }}</ref> has shown that there are no ribosomal proteins close to the reaction site for polypeptide synthesis. This suggests that the protein components of ribosomes do not directly participate in peptide bond formation catalysis, but rather that these proteins act as a scaffold that may enhance the ability of rRNA to synthesize protein (see: [[Ribozyme]]).

[[Image:010 small subunit-1FKA.gif|thumb|Molecular structure of the 30S subunit from ''[[Thermus thermophilus]]''.<ref name="Wimberly-2000"/> Proteins are shown in blue and the single RNA chain in brown.]]

The ribosomal subunits of [[prokaryote]]s and [[eukaryote]]s are quite similar.<ref name="Alberts-2002">{{cite book | vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P | chapter = Membrane-bound Ribosomes Define the Rough ER | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26841/#A2204 | title = Molecular Biology of the Cell | edition = 4th | location = New York | publisher = Garland Science | date = 2002 | isbn = 978-0-8153-4072-0 | pages = 342 }}</ref>

The unit of measurement used to describe the ribosomal subunits and the rRNA fragments is the [[Svedberg]] unit, a measure of the rate of [[sedimentation]] in [[centrifugation]] rather than size. This accounts for why fragment names do not add up: for example, bacterial 70S ribosomes are made of 50S and 30S subunits.

Prokaryotes have 70[[Svedberg|S]] ribosomes, each consisting of a small ([[30S]]) and a large ([[50S]]) subunit. ''E. coli'', for example, has a [[16S ribosomal RNA|16S]] RNA subunit (consisting of 1540 nucleotides) that is bound to 21 proteins. The large subunit is composed of a [[5S ribosomal RNA|5S]] RNA subunit (120 nucleotides), a 23S RNA subunit (2900 nucleotides) and 31 [[protein]]s.<ref name="Alberts-2002" />

:{| class="wikitable float-right" style="text-align:center"
|+ Ribosome of ''E. coli'' (a bacterium)<ref name="Garrett-2009" />{{rp|962}}
|-
! width="25%"| ribosome
! width="25%"| subunit
! width="25%"| rRNAs
! width="25%"| r-proteins
|-
| rowspan="3" | 70S || rowspan="2" | 50S || 23S (2904 [[Nucleotide|nt]]) || rowspan="2" | 31
|-
| 5S (120 nt)
|-
| 30S || 16S (1542 nt) || 21
|}

Affinity label for the tRNA binding sites on the ''E. coli'' ribosome allowed the identification of A and P site proteins most likely associated with the peptidyltransferase activity;<ref name="Tirumalai-2021a"/> labelled proteins are L27, L14, L15, L16, L2; at least L27 is located at the donor site, as shown by E. Collatz and A.P. Czernilofsky.<ref>{{cite journal | vauthors = Collatz E, Küchler E, Stöffler G, Czernilofsky AP | title = The site of reaction on ribosomal protein L27 with an affinity label derivative of tRNA Met f | journal = FEBS Letters | volume = 63 | issue = 2 | pages = 283–6 | date = April 1976 | pmid = 770196 | doi = 10.1016/0014-5793(76)80112-3 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Czernilofsky AP, Collatz EE, Stöffler G, Kuechler E | title = Proteins at the tRNA binding sites of Escherichia coli ribosomes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 71 | issue = 1 | pages = 230–4 | date = January 1974 | pmid = 4589893 | pmc = 387971 | doi = 10.1073/pnas.71.1.230 | bibcode = 1974PNAS...71..230C | doi-access = free }}</ref> Additional research has demonstrated that the S1 and S21 proteins, in association with the 3′-end of 16S ribosomal RNA, are involved in the initiation of translation.<ref>{{cite journal | vauthors = Czernilofsky AP, Kurland CG, Stöffler G | title = 30S ribosomal proteins associated with the 3'-terminus of 16S RNA | journal = FEBS Letters | volume = 58 | issue = 1 | pages = 281–4 | date = October 1975 | pmid = 1225593 | doi = 10.1016/0014-5793(75)80279-1 | doi-access = free }}</ref>