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23S ribosomal RNA is the large ribosomal RNA component of the 50S large ribosomal subunit in bacteria and archaea. It forms much of the subunit's structural framework and the RNA-based environment of the peptidyl transferase center. The functional RNA is distinct from the DNA sequence from which it is transcribed: a genomic sequence record and a mature ribosomal component describe different physical molecules.^[ref]

Structural experiments with an archaeal large ribosomal subunit placed two substrate analogues against conserved regions of 23S ribosomal RNA. The absence of nearby protein side chains at the bond-forming position supported the conclusion that the ribosome is a ribozyme.^[ref]

The 23S label refers to a sedimentation coefficient, not 23 nucleotides. It must not be added as though it were a molecular mass: the 50S large ribosomal subunit contains 23S ribosomal RNA, 5S ribosomal RNA and ribosomal proteins, and its sedimentation depends on the assembled object's shape and resistance to motion.^[ref] Thus a measurement of an isolated RNA component and a measurement of the complete subunit need not carry the same S value.

The 30S ribosomal subunit is the bacterial small ribosomal subunit, an assembly containing 16S ribosomal RNA and ribosomal proteins. It binds messenger RNA and helps position transfer RNA anticodons relative to the message during biological translation. The large ribosomal subunit supplies the peptidyl transferase center; a small subunit alone is not a complete elongating ribosome.^[ref]

The 30S label refers to a sedimentation coefficient, not a count of 30 molecules or a molecular mass of 30 units. A svedberg is a unit of that transport coefficient. Shape, mass and interaction with the surrounding liquid all affect sedimentation.^[ref]

A short reading-frame example explains the subunit's positioning role. Grouping AUGAAACCC from its first base gives AUG|AAA|CCC. Shifting the grouping by one base gives UGA|AAC|CC, with an incomplete final group. The nucleotide inventory is unchanged, but the triplets presented for decoding differ. Position and pairing therefore matter alongside simple RNA attachment.

30S initiation complex experiments examine an assembled state involving the subunit, message and initiator transfer RNA. Reconstituting the subunit with or without ribosomal protein S1 has separated message binding from productive placement in the decoding channel.^[ref]

The large ribosomal subunit is the ribosomal subunit containing the peptidyl transferase center, where peptide bonds are formed during biological translation. It contains ribosomal RNA and ribosomal proteins, and provides a ribosomal exit tunnel through which the growing polypeptide passes. The small ribosomal subunit organizes decoding of messenger RNA; synthesis requires coordination between both parts of the ribosome.^[ref]

For a reaction example, let one peptidyl-tRNA carry a chain of four amino acid residues and an incoming aminoacyl-tRNA carry one more. Peptidyl transfer produces a five-residue chain attached to the incoming transfer RNA. The existing chain is transferred and extended; the reaction does not join five free amino acids all at once. Subsequent movement of the ribosome prepares the complex for another elongation cycle.

A 2000 structural experiment examined an archaeal large ribosomal subunit and complexes containing two substrate analogues. The analogues contacted conserved 23S ribosomal RNA in the catalytic region, with no protein side-chain atoms within about 18 angstroms of the bond-forming position. This supported the identification of the ribosome as a ribozyme. The same study found that both RNA and proteins contribute to the ribosomal exit tunnel.^[ref] The structural evidence identifies the reaction environment; it does not make every chemical step or proton-transfer mechanism directly visible.

The peptidyl transferase center is the catalytic region of the large ribosomal subunit where a growing peptide is transferred between transfer RNAs during translation. An amino group on the incoming aminoacyl-tRNA attacks the ester linking the existing chain to the P-site tRNA. A new peptide bond forms, leaving the extended chain attached to the A-site tRNA. Thus the reaction transfers a chain between tRNAs rather than joining two free amino acids in solution.^[ref]

For example, if the P-site tRNA carries a three-residue chain and the A-site tRNA carries one amino acid, the product is a four-residue chain on the A-site tRNA plus a deacylated P-site tRNA. Subsequent ribosomal movement places that peptidyl-tRNA for another round. The peptidyl transferase center performs the bond-forming step; decoding and translocation are coordinated but distinct functions of the ribosome.

A decisive structural experiment placed substrate analogues in crystals of an archaeal large subunit. The analogues occupied a cavity formed by conserved ribosomal RNA; in those structures the nearest protein side-chain atoms were about 18 angstroms from the bond-forming position. This strongly supported an RNA-based catalytic center and the description of the ribosome as a ribozyme.^[ref]

The result identifies the catalytic environment, but a static structure alone does not determine every proton-transfer step or every kinetic contribution of neighboring groups. Structural placement, chemical perturbations and reaction-rate measurements answer complementary mechanistic questions.

The ribosomal A site, or aminoacyl site, is the ribosome position that accepts an incoming aminoacyl-tRNA during biological translation. Its anticodon is tested against a messenger RNA codon at the small ribosomal subunit, while its amino-acid-bearing end reaches the peptidyl transferase center of the large ribosomal subunit. The A site therefore spans coordinated decoding and chemical positions rather than being a hole in only one subunit.^[ref]

For an original chain-transfer example, a peptidyl-tRNA at the ribosomal P site carries eight amino acid residues, and an A-site aminoacyl-tRNA carries one. Peptidyl transfer produces a nine-residue chain attached to the A-site carrier. Movement of the ribosome then relocates the carriers and presents a new codon for the next incoming transfer RNA. Entry, bond formation and movement are distinct stages.

Occupancy also depends on the stage of biological translation. During bacterial initiation, bacterial initiation factor IF1 binds in the small-subunit A site instead of an elongating aminoacyl-tRNA. A primary X-ray crystallography study found IF1 occluding that position and changing nearby 16S ribosomal RNA bases.^[ref] This explains why observing a bound molecule at the A site does not by itself identify an elongation-ready complex.

The ribosomal exit tunnel is the passage through the large ribosomal subunit leading from the peptidyl transferase center toward the exterior of the ribosome. The growing polypeptide enters this passage during biological translation. Its walls contain ribosomal RNA together with contributions from ribosomal proteins; the tunnel is part of the molecular machine, rather than an opening through the cell membrane.^[ref]

The tunnel can also help distinguish particular nascent sequences. A 2002 experiment identified a SecM peptide motif that arrests elongation. Mutations in 23S ribosomal RNA or ribosomal protein L22 could bypass this arrest; the affected residues faced the tunnel's constriction. Changing tunnel components changed the response to the nascent product, supporting a functional interaction between the polypeptide and its surroundings.^[ref]

Consider two chains containing the same number of amino acid residues in different orders. Equal length alone would not imply identical passage or stalling behaviour, because their side chains encounter different local environments. The SecM result makes that distinction experimentally meaningful: the ribosomal exit tunnel can participate in sequence-dependent regulation, while remaining the route followed by many unrelated polypeptides.

The ribosomal P site, or peptidyl site, is the ribosome binding position associated with the transfer RNA carrying the growing peptide before a peptide-transfer step. During initiation, the initiator transfer RNA is placed there directly, paired with the start codon of the messenger RNA. Its decoding contacts involve the small ribosomal subunit, while its acceptor end extends toward the peptidyl transferase center in the large ribosomal subunit.^[ref]

For an original chain-length example, let a P-site peptidyl-tRNA carry four amino acid residues, and let an aminoacyl-tRNA at the ribosomal A site carry one. Forming the next peptide bond transfers the four-residue chain onto the incoming residue: the five-residue product is initially attached to the A-site carrier, leaving the P-site carrier deacylated. Subsequent movement positions the carriers for another cycle.^[ref]

Thus “P site” names a position in a changing reaction cycle. It does not imply that a peptide remains permanently attached to whichever transfer RNA first occupied that position.

Shijie Huang is the first author of a 2020 study testing why 5S ribosomal RNA is normally an independent molecule within the ribosome. The researchers engineered a covalent fusion of rearranged 5S RNA with 23S RNA and produced bacterial cells without free 5S RNA.^[ref]

The engineered cells remained viable under the tested standard conditions, and their fully assembled ribosomes could perform translation. However, they accumulated malformed large subunits that failed to form stable complete ribosomes. Cryo-electron microscopy revealed defects near the peptidyl transferase center in these misassembled particles.^[ref]

This separates two questions: whether an altered ribosome can function once assembled, and whether it assembles efficiently. The results support an assembly-related reason for preserving an independent 5S RNA. Viability in one laboratory condition does not establish that the fusion would be equally successful across environments or evolutionary timescales.

A substrate analogue is a molecule designed or recognized as resembling an enzyme's substrate in features relevant to binding or reaction. Resemblance may involve molecular shape, electric charge or the arrangement of reactive groups. An analogue need not reproduce every property of the natural substrate: it may bind without undergoing the same reaction, react at a different rate or stabilize a useful experimental complex.

For a conceptual example, suppose a natural substrate is recognized through two binding groups and transformed at a third group. An analogue retaining the recognition groups but replacing the reactive group could reveal how the active site holds the substrate. Observing binding would answer a recognition question; it would not establish the natural reaction rate. Nor does the word analogue alone guarantee competitive inhibition: that requires evidence about how the compound affects the enzyme under the tested conditions.

In a 2000 ribosome study, structures of the large ribosomal subunit bound to two substrate analogues located the peptidyl transferase center. Conserved 23S ribosomal RNA contacted the analogues, providing evidence about the chemical environment for peptide bond synthesis.^[ref] The experimental value lies in making a normally transient arrangement accessible to structural measurement; interpreting the natural substrate still requires attention to what the analogue preserves and changes.