Revisiting the Closed-Loop Model and the Nature of mRNA 5'-3' Communication.

Communication between the 5' and 3' ends of mature eukaryotic mRNAs lies at the heart of gene regulation, likely arising at the same time as the eukaryotic lineage itself. Our view of how and why it occurs has been shaped by elegant experiments that led to nearly universal acceptance of the "closed-loop model." However, new observations suggest that this classic model needs to be reexamined, revised, and expanded. Here, we address fundamental questions about the closed-loop model and discuss how a growing understanding of mRNA structure, dynamics, and intermolecular interactions presents new experimental opportunities. We anticipate that the application of emerging methods will lead to expanded models that include the role of intrinsic mRNA structure and quantitative dynamic descriptions of 5'-3' proximity linked to the functional status of an mRNA and will better reflect the messy realities of the crowded and rapidly changing cellular environment.