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).
The mechanism by which these large mRNA-protein (mRNP) complexes then move through dense nucleoplasm to reach the nuclear pores has been the subject of intense study and speculation (2, 3).
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).
This solid-state transport model is supported by observations made in fixed nuclei that show some transcripts distributed along tracks that originate from the locus of the parent gene (5, 6).
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 idea is supported by observations that certain mRNAs exit from one side of the nucleus (8) and that, in yeast, many transcriptionally active gene loci are located near the nuclear periphery (9).
By contrast, a number of other studies have found that mRNP complexes move quite freely within the nucleus (10–16).
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).