Circularization and Ribosome Recycling: From Polysome Topology to Translational Control.

It has been known for decades that eukaryotic cellular mRNAs are frequently translated by multiple ribosomes organized into polysomes of diverse topology, including circular arrangements. The closed-loop model, in which the 5' cap and 3' poly(A) tail are bridged by initiation factors, provided a mechanistic basis for mRNA circularization and suggested that the spatial proximity of termini facilitates ribosome recycling. Various biochemical, structural, and imaging approaches-including electron microscopy, atomic force microscopy, cryo-electron tomography, and single-molecule fluorescence-have since demonstrated that polysomes indeed adopt compact and heterogeneous conformations, with circular assemblies representing a significant fraction. Although direct visualization of ribosome recycling remains technically challenging, ribosome turnover experiments, kinetic analyses and modeling support the concept of closed-loop-assisted reinitiation (CLAR), whereby terminating ribosomes are re-utilized to sustain translation efficiency. Together, the findings suggest that mRNA circularization is a dynamic and regulated state that enhances protein synthesis under specific conditions, while linear or modular polysome architectures may dominate in others. Understanding the balance between these modes of translation remains central to elucidating the interplay between mRNA topology, ribosome dynamics, and translational control.