In addition to the association of transcriptional activation with large-scale chromatin decondensation, early in our work we also noticed a change in nuclear positioning accompanying transcriptional activation. This was first observed in our first generation experiments when we tethered the VP16 acidic activation domain to a heterochromatic amplified chromosome region, A03. The A03 amplified chromosome region, normally located near the nuclear periphery, not only unfolded dramatically but changed to a more interior position. In our second generation approach, we noticed that activation of the beta-globin promoter during induced differentiation of MEL cells also induced a more interior position to a plasmid transgene array carrying many copies of a reporter gene driven by a beta-globin mini-LCR and promoter sequences. To examine this more closely, we showed that tethering the acidic activation domain of VP16 to a peripherally associated plasmid transgene repeat caused this chromosome site to move to the nuclear interior. This ability of VP16 to induce a peripherally localized chromosome site to change to a more interior position was observed for other acidic activators, such as p53. It also was observed for transcriptionally dead VP16 mutants. We used an inducible tethering system to show that this change in nuclear positioning could be induced within 1-2 hrs after tethering and that it occurred through a rapid movement over just several minutes. This curvilinear movement was shown to be dependent on actin and nuclear myosin 1c. The induced changes in nuclear positioning described above were observed with artificial engineered chromosome regions.
More recently we have used a BAC transgene system to demonstrate the association with nuclear speckles of the Hsp70 gene locus after heat shock induction of transcription. Initial investigation has indicated that this nuclear speckle association can occur through association with nearby pre-existing speckles, nucleation of new speckles near the transgene, or in a fraction of nuclei through fast, long-range chromosomal movement to a pre-existing speckle.
A technical difficulty complicating study of these long range interphase chromosome movements is that these long-range movements are extremely photosensitive. We are currently developing new cell lines and have acquired new microscopes which should facilitate re-examination of these long range chromosome movements.
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Chuang CH, Carpenter AE, Fuchsova B, Johnson T, de Lanerolle P, Belmont AS. Long-range directional movement of an interphase chromosome site. Curr Biol. 2006 Apr 18; 16 (8) :825-31. PubMed PMID:16631592.
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