Dundr et al. report. The study is the first to show directed movement of mammalian genes that are being transcribed.
Interphase chromosomes jiggle, but they usually remain within so-called territories. Particular DNA segments, however, can travel substantial distances. One situation that might involve DNA movement is the liaison between active genes and Cajal bodies, which harbor small nuclear RNAs (snRNAs) for splicing. Cajal bodies often show up near working genes for snRNA and histone proteins, although researchers didn't know whether Cajal bodies form near these genes or whether the partners move toward each other.
To find out, Dundr et al. inserted into HeLa cells an artificial chromosome carrying 16 copies of an snRNA gene. The team tracked the positions of the chromosome and Cajal bodies after the genes started transcription. The two components cozied up, the researchers found. The Cajal bodies were sluggish, remaining in roughly the same place. The DNA, by contrast, was responsible for most of the movement, particularly during a final lunge that began around six to seven hours after gene activation. In total, it traveled about two to three microns.
The researchers also found that a tether of RNA linked an active gene to the Cajal body, indicating that the newly made strand was feeding directly into the structure. Disrupting actin, Dundr et al. showed, prevented the movement, suggesting that actin helps haul certain active genes to Cajal bodies. The question of why this movement occurs remains unanswered.