1 0 2 3 a r t i c l e s RESULTS Pax3 binds to major satellite repeatsTo examine transcription factors operating at heterochromatic regions, we first investigated transcription factors that had been shown to associate with heterochromatic core components 15 . Pax3 emerged as a particularly notable candidate because it has been reported to interact with both heterochromatin protein-1 (HP1) and the transcriptional regulator Kap1(Trim28) 16 . In addition, subrepeat Heterochromatin is important for genome integrity and stabilization of gene-expression programs. We have identified the transcription factors Pax3 and Pax9 as redundant regulators of mouse heterochromatin, as they repress RNA output from major satellite repeats by associating with DNA within pericentric heterochromatin. Simultaneous depletion of Pax3 and Pax9 resulted in dramatic derepression of major satellite transcripts, persistent impairment of heterochromatic marks and defects in chromosome segregation. Genome-wide analyses of methylated histone H3 at Lys9 showed enrichment at intergenic major satellite repeats only when these sequences retained intact binding sites for Pax and other transcription factors. Additionally, bioinformatic interrogation of all histone methyltransferase Suv39h-dependent heterochromatic repeat regions in the mouse genome revealed a high concordance with the presence of transcription factor binding sites. These data define a general model in which reiterated arrangement of transcription factor binding sites within repeat sequences is an intrinsic mechanism of the formation of heterochromatin.npg

Heterochromatin protein 1 (HP1) is a central factor in establishing and maintaining the repressive heterochromatin state. To elucidate its mobility and interactions, we conducted a comprehensive analysis on different time and length scales by fluorescence fluctuation microscopy in mouse cell lines. The local mobility of HP1alpha and HP1beta was investigated in densely packed pericentric heterochromatin foci and compared with other bona fide euchromatin regions of the nucleus by fluorescence bleaching and correlation methods. A quantitative description of HP1alpha/beta in terms of its concentration, diffusion coefficient, kinetic binding, and dissociation rate constants was derived. Three distinct classes of chromatin-binding sites with average residence times t(res)

In the version of this article initially published, the colors in the key in Figure 7c were incorrectly reversed. Red should have represented telomeres and gray a-satellite regions. The error has been corrected in the HTML and PDF versions of the article.