Tracking transcripts

mRNAs do not travel willy-nilly through the nucleus, according to Molenaar et al. (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...