page 191). Instead, they are transported by an energy-dependent mechanism that may bring them to quality control sites before they are exported to the cytoplasm.
Random diffusion from transcription sites to nuclear pores was generally accepted as the travel mode of preference for polyadenylated mRNAs. Based on the high mobility of oligo(dT) probes, transcripts were assumed to be moving through the nucleoplasm at rates comparable to diffusion. But Molenaar and colleagues find that these speeds were probably overestimates resulting from free probe. Using a tighter-binding oligo(U) probe, they find that mRNA moves 10-fold more slowly than previous estimates.
This movement is energy dependent, indicating that an active process transports the mRNAs, perhaps by a motor or along chromatin fibers. The group now plans to inhibit nuclear mRNA-binding proteins to identify those that are essential for transport.
Transcripts were mobile even at speckles—putative nuclear storage sites for RNA-processing enzymes. This dynamic association suggests that transcripts are not important structural elements of speckles. Most transcripts passed through speckles at least briefly, so transcripts might instead be sent there to be checked for proper splicing. Since speckles contain splicing factors, they might even fix mRNAs that fail inspection, thus accounting for the small fraction of transcripts that were immobile in speckles. ▪