Wilhelm bernhard workshop on the cell nucleus

NUCLEOLUS


UBF INDUCES CHROMATIN REMODELING

Chen D.¹, Belmont A. S. ², and Huang S. ¹

¹Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, Illinois, USA;  ²Department of Cell and Structural Biology, University of Illinois, Urbana-Champaign, USA.
The upstream binding factor (UBF) is involved in RNA polymerase I transcription.  In addition to its binding to the promoter, it has recently been shown to associate with sequences across the rDNA repeats (O'Sullivan et al., 2002).  We were interested in determining whether UBF also plays a role in chromatin remodeling.  We used the A03_1 cell line containing a ~90 Mbp, condensed, late replicating chromosome region labeled lac operator repeats.  This region appears as a condensed spot throughout most of the interphase cell cycle.  A fusion protein of lac repressor and a strong transactivator, VP16, localizes to the lac operator repeats and induces large-scale chromatin decondensation (Tumbar et al., 1999).  We have fused UBF1 to the lac repressor and found that targeting UBF1 to the lac operator repeats also induces chromatin decondensation.  The UBF fusion protein appears to behave similarly as endogenous UBF, including localization to the nucleolus and dimerizing with wild type UBF, but have much stronger affinity to the lac operator repeats.  The decondensed chromatin induced by UBF targeting colocalizes with acetylated core histones and some chromatin remodeling proteins.  These findings demonstrate that association of UBF to condensed repetitive chromatin is sufficient to induce chromatin remodeling, suggesting a novel role of UBF in regulation of rDNA transactivation.