Polyribosomes

A polyribosome (or polysome) is a complex of an mRNA molecule and two or more ribosomes that act to translate mRNA instructions into polypeptides. Originally coined "ergosomes" in 1963, they were further characterized by Jonathan Warner, Paul M. Knopf, and Alex Rich. The simultaneous analysis of the polysome with all the peptides it produces under given conditions (the translatome) lead to a better insight of the complexity of how mutations, extracellular stimuli, intercellular cues, growth conditions, and stress could lead to the change of translation in the cell. Polysomes consist of varying numbers of ribosomes, and each ribosome contributes to the addition of its substantial mass. They act as a platform for signalling molecules to get involved in the emergence of polypeptides and ribosomes, and aid the nascent polypeptides to fold in the presence of chaperones, which later impact on the function of synthesized polysomes.

Atomic-force microscopy (AFM) was used to obtain the images of polysomes from mouse brain samples in air and liquid. The images were then reconstructed to depict structures in three-dimensions, thereby facilitating the study into how polysomes are organized. Polysomes can be directly visualized by electron microscopy because they form very high molecular weight particles. Many ribosomes simultaneously read one mRNA polypeptide by progressing along the mRNA chain to synthesize the same protein. They may appear as linear polysomes or circular rosettes with microscopy, but are mainly circular in vivo. This circularization is aided by the fact that mRNA is able to be twisted into a circular formation, allowing a cycle of rapid ribosome recycling to occur. The 5' 7-methylguanosine cap and 3' poly(A) tail present on eukaryotic mRNA aid in this process.

Polyribosomes can be found in three forms: free, cytoskeletal bound, and membrane bound. The three forms differ by the poly(A) and non-poly(A) protein binding sequence.

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