Polysome

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. Polysome activity can be used to measure the level of gene expression through a technique called polysomal profiling.

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. Different cells produce different structures of polysomes.

Prokaryotic

Bacterial polysomes have been found to form 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. 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 is observed.

Polysomes are present in archaea, but not much is known about the structure.

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.

In vitro

Atomic force microscopy used in in vitro studies have shown that circular eukaryotic polysomes can be formed by free polyadenylated mRNA in the presence of initiation factor eIF4E bound to the 5' cap and PABP bound to the 3'-poly(A) tail. However, this interaction between cap and the poly(A)-tail mediated by a protein complex is not a unique way of circularizing polysomal mRNA. It has been found that topologically circular polyribosomes can be successfully formed in the translational system with mRNA with no cap and no poly(A) tail as well as a capped mRNA without a 3'-poly(A) tail.

Membrane-bound

Polyribosomes bound to membranes are restricted by a 2 dimensional space given by 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 a smooth pathway. Each ribosome is turned relative to the previous one, resembling a planar spiral.