Compaction of polyribosomal mRNA.

RNA molecules may form compact secondary and tertiary structures in order to exert their functions. Their folding is assisted by various RNA-binding proteins and was found to be critical for the functionality of many non-coding RNAs (e.g., tRNA and rRNA). For messenger RNA, however, only little evidence was found for compaction to occur while it is being translated. One well-studied mode of compaction is through the interaction between the two ends of mRNA, which give rise to the so-called 'circular mRNA.' Recently, work from several labs revealed that translated mRNAs are compacted far beyond the circular structure. Polyribosomes formed by a continuous cell-free translation system were found by electron microscopy to form a double-row structure in which small subunits of non-adjacent ribosomes are in close proximity. A reconstituted 3D structure of polysomes, which was derived from high-resolution cryoelectron tomography (CET) of E. coli extracts, also revealed compact structures in which the large subunits were oriented away from each other. Co-sedimentation and co-precipitation analyses revealed that the 3' UTR of many eukaryotic mRNAs are in strong association with the coding regions, presumably through interaction with translating ribosomes. Taken together, these data imply that in vivo, polyribosomal mRNAs are found in a compact structure in which distant domains are in close contact. How these interactions affect the progress of ribosomes along the coding region, and whether these interactions have regulatory implications are yet to be determined.