After mRNAs are synthesized, processed, and become associated with a number of different proteins at the transcription site, they are released into the nucleoplasm (1).
Early workers proposed that mRNP complexes are transferred along a chain of receptors until they reach a nuclear pore, expending metabolic energy in the process (4).
A second theory, called the “gene-gating” hypothesis, proposes that active genes are situated near the nuclear periphery and that mRNAs exit the nucleus through the nearest pores (7).
This view is supported by studies of the distribution of newly synthesized Balbiani ring RNA in the salivary gland cells of insects (11),
fluorescence recovery after photobleaching and fluorescence correlation spectroscopy studies of probes that bind to the poly(A) tails of mRNAs (12–15), and from single-particle analysis of mRNP complexes bound to GFP-linked proteins (16).