thumb|Several ribosomes synthesizing a polypeptide on the same mRNA strand
A polysome (or polyribosome or ergosome) is a group of ribosomes bound to an mRNA molecule like "beads" on a "thread". It consists of 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.
Polysomes are formed during the elongation phase when ribosomes and elongation factors synthesize the encoded polypeptide. Multiple ribosomes move along the coding region of mRNA, creating a polysome. The ability of multiple ribosomes to function on an mRNA molecule explains the limited abundance of mRNA in the cell. Polyribosome structure differs between prokaryotic polysomes, eukaryotic polysomes, and membrane bound polysomes.
Structure
Electron microscopy technologies such as staining, metal shadowing, and ultra-thin cell sections were the original methods to determine polysome structure. The development of cryo-electron microscopy techniques has allowed for increased resolution 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. An investigation of the ratio of polyribosomal shape elucidated that a high number of circular and zigzag polysomes were found after several rounds of translation. A longer period of translation caused the formation of densely packed 3-D helical polysomes.
Eukaryotic
Studies have shown that eukaryotic polysomes exhibit linear configurations. Densely packed 3-D helices and planar double-row polysomes were found with variable packing 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 packed left-handed helices with four ribosomes per turn". This dense packing can determine their function as regulators of translation, with 3-D polyribosomes being found in sarcoma cells using fluorescence microscopy. The data from polysomal profiling showed that host mRNAs are outcompeted by viral mRNAs for polysomes, therefore decreasing the translation of host mRNA and increasing the translation of viral mRNA.