Polyribosomer är en aggregering av

Ribosomes bound to an mRNA molecule

This article fryst vatten about the begrepp in fängelse biology. For the begrepp in crystallography, see Polysome (crystallography).

A polyribosome (or polysome or ergosome) fryst vatten a group of ribosomes bound to an mRNA molekyl like “beads” on a “thread”.^[1] It consists of a complex of an mRNA molekyl and two or more ribosomes that act to translatemRNA instructions into polypeptides.

Originally coined "ergosomes" in , they were further characterized bygd Jonathan Warner, Paul M. Knopf,^[2] and Alex rik.

Polysomes are formed during the elongation phase when ribosomes and elongation factors synthesize the encoded polypeptide. Multiple ribosomes move along the coding distrikt of mRNA, creating a polysome.

The ability of multiple ribosomes to function on an mRNA molekyl explains the limited överflöd of mRNA in the cell.^[3] Polyribosome structure differs between prokaryotic polysomes, eukaryotic polysomes, and membrane bound polysomes.^[1] Polysome activity can be used to measure the level of gene expression through a technique called polysomal profiling.^[4]

Structure

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Electron microscopy technologies such as staining,^[5] metall shadowing,^[6] and ultra-thin fängelse sections were the original methods to determine polysome structure.

The development of cryo-electron microscopy techniques has allowed for increased upplösning of the image, leading to a more precise method to determine structure. Different structural configurations of polyribosomes could reflect a variety in translation of mRNAs.

Several ribosomes synthesizing a polypeptide on the same mRNA strand

An investigation of the ratio of polyribosomal shape elucidated that a high number of rund and zigzag polysomes were funnen after several rounds of translation. A längre period of translation caused the formation of densely packad 3-D helical polysomes.^[1] Different cells tillverka different structures of polysomes.

Prokaryotic

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Bacterial polysomes have been funnen to struktur double-row structures. In this conformation, the ribosomes are contacting each other through smaller subunits. These double row structures generally have a “sinusoidal” (zigzag) or 3-D helical path.

[1] Ibland används begreppet specifikt om det steg, translationen, där RNA översätts till sekvenser av aminosyror i cellens ribosomer, men oftast åsyftas en flerstegsprocess som börjar med transkription och slutar med translationen

In the “sinusoidal” path, there are two types of contact between the small subunits- “top-to-top” or “top-to-bottom”. In the 3-D helical path, only “top-to-top” contact fryst vatten observed.^[1]

Polysomes are present in archaea, but not much fryst vatten known about the structure.^[7]

Eukaryotic

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In cells

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in situ (in cell) studies have shown that eukaryotic polysomes exhibit linear configurations.

Densely packad 3-D helices and planar double-row polysomes were funnen with variabel förpackning including “top-to-top” contacts similar to prokaryotic polysomes. Eukaryotic 3-D polyribosomes are similar to prokaryotic 3-D polyribosomes in that they are “densely packad left-handed helices with fyra ribosomes per turn”.

Proteinsyntesen skiljer sig något mellan

This dense förpackning can determine their function as regulators of translation, with 3-D polyribosomes being funnen in sarcoma cells using fluorescence microscopy.^[1]

Cell free

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Atomic force microscopy used in in vitro studies have shown that rund eukaryotic polysomes can be formed bygd free polyadenylated mRNA in the presence of initiation factor eIF4E bound to the 5’ cap and PABP bound to the 3’-poly(A) svans.

However, this interaction between cap and the poly(A)-tail mediated bygd a protein complex fryst vatten not a unique way of circularizing polysomal mRNA. It has been funnen that topologically rund polyribosomes can be successfully formed in the translational struktur with mRNA with no cap and no poly(A) svans as well as a capped mRNA without a 3’-poly(A) tail.^[1]

Membrane-bound

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Polyribosomes bound to membranes are restricted bygd a 2 dimensional space given bygd the membrane surface.

The restriction of inter-ribosomal contacts causes a round-shape configuration that arranges ribosomes along the mRNA so that the entry and exit sites form eller gestalt a smooth pathway. Each ribosome fryst vatten turned relative to the previous one, resembling a planar spiral.^[1]

Profiling

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Polysomal profiling fryst vatten a technique that uses cycloheximide to fängelse translation and a sucrose gradient to separate the resulting fängelse extrakt bygd centrifugation.^[3] Ribosome-associated mRNAs migrate faster than free mRNAs and polysome associated mRNAs migrate faster than ribosome associated mRNAs.

Several peaks corresponding to mRNA are revealed bygd the measurement of total protein across the gradient. The corresponding mRNA fryst vatten associated with increasing numbers of ribosomes as polysomes. The presence of mRNA across the gradient reveals the translation of the mRNA. Polysomal profiling fryst vatten optimally applied to cultured cells and tissues to track the translational ställning eller tillstånd of an identified mRNA as well as measure ribosome density.^[4] This technique has been used to compare the translational ställning eller tillstånd of mRNAs in different fängelse types.

For example, polysomal profiling was used in a study to investigate the effect of vesicular stomatitis virus (VSV) in mammalian cells.^[8] The information from polysomal profiling showed that host mRNAs are outcompeted bygd viral mRNAs for polysomes, therefore decreasing the translation of host mRNA and increasing the translation of viral mRNA.^[8]

References

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External links

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