Polysome

Polysomes, also called polyribosomes, are complexes composed of multiple ribosomes simultaneously translating a single messenger RNA molecule. They form during protein synthesis when several ribosomes are attached end‑to‑end along the length of an mRNA strand. Each ribosome in the polysome reads the codons on the mRNA and adds amino acids to the nascent polypeptide chain, enabling rapid production of many identical proteins from a single mRNA transcript.

Structurally, a polysome contains an RNA scaffold with ribosomal subunits spaced roughly one ribosome length apart. The spacing allows ribosomes to move along the mRNA without collision, and cross‑linking proteins can stabilize the arrangement. Electron‑microscopy images reveal the ribosomes as large dark spheres clustered around the mRNA, a typical appearance in cell extracts.

Polysomes are a key indicator of active translation within a cell. Their abundance correlates with the rate of protein synthesis and can be modulated by cellular signaling pathways, stress responses, or nutrient availability. Changes in polysome profiles are often used to study the regulation of gene expression and the effects of drugs on translation.

Polysomes are isolated by ultracentrifugation through sucrose or glycerol gradients, which separate complexes based on density. Ribosomal subunits and polysomes migrate to distinct fractions, and their distribution can be analyzed by UV absorbance or by translating ribosome profiling. Cryo‑electron microscopy and high‑resolution mass spectrometry are increasingly employed to examine polysome composition and post‑translational modifications.

Research on polysomes has revealed their involvement in genetic disorders and cancer. Aberrant translation initiation or ribosome recruitment can lead to dysregulated protein production, contributing to disease pathology. Therefore, polysome profiling serves as a diagnostic and therapeutic tool in translational medicine.