We combine interferometric detection of single gold nanoparticles, single molecule microscopy, and fluorescence lifetime measurement to study the modification of the fluorescence decay rate of an emitter close to a nanoparticle. In our experiment, gold particles with a diameter of 15 nm were attached to single dye molecules via double-stranded DNA of different lengths. Nanoparticle-induced lifetime modification (NPILM) has promise in serving as a nanoscopic ruler for the distance range well beyond 10 nm, which is the upper limit of fluorescence resonant energy transfer (FRET). Furthermore, the simultaneous detection of single nanoparticles and fluorescent molecules presented in this work provides new opportunities for single molecule biophysical studies.

We describe the isolation and characterization of Friend of Prmt1 (Fop), a novel chromatin target of protein arginine methyltransferases. Human Fop is encoded by C1orf77, a gene of previously unknown function. We show that Fop is tightly associated with chromatin, and that it is modified by both asymmetric and symmetric arginine methylation in vivo. Furthermore, Fop plays an important role in the ligand-dependent activation of estrogen receptor target genes, including TFF1 (pS2). Fop depletion results in an almost complete block of estradiol-induced promoter occupancy by the estrogen receptor. Our data indicate that Fop recruitment to the promoter is an early critical event in the activation of estradiol-dependent transcription.

Locus control regions (LCRs) alleviate chromatinmediated transcriptional repression. Incomplete LCRs partially lose this property when integrated in transcriptionally restrictive genomic regions such as centromeres. This frequently results in position effect variegation (PEV), i.e. the suppression of expression in a proportion of the cells. Here we show that this PEV is in¯uenced by the heterochromatic protein SUV39H1 and by the Polycomb group proteins M33 and BMI-1. A concentration variation of these proteins modulates the proportion of cells expressing human globins in a locus-dependent manner. Similarly, the transcription factors Sp1 or erythroid Kru È ppel-like factor (EKLF) also in¯uence PEV, characterized by a change in the number of expressing cells and the chromatin structure of the locus. However, in contrast to results obtained in a euchromatic locus, EKLF in¯uences the expression of the g-more than the b-globin genes, suggesting that the relief of silencing is caused by the binding of EKLF to the LCR and that genes at an LCR proximal position are more likely to be in an open chromatin state than genes at a distal position. Keywords: gene activation/position effects/transcription IntroductionThe observation that the presence of the b globin locus control region (LCR) results in copy number-dependent expression of a globin transgene, independent of its position of integration in the host genome, led to the proposal that LCR function includes the establishment and/or maintenance of an open chromatin structure (Grosveld et al., 1987;Festenstein et al., 1996;Milot et al., 1996). Partial LCRs have lost this property and are very sensitive to position effects, particularly when integrated into transcriptionally repressive regions in the genome of transgenic mice (reviewed in Kioussis and Festenstein, 1997). Such integration leads to expression in only a proportion of the cells, which is similar to classical position effect variegation (PEV) (reviewed in Karpen, 1994) or to a novel cell timing position effect, in which all cells express but for only part of the time (Milot et al., 1996). The PEV expression was found in mice containing a deletion of the hypersensitive site 2 (HS2) of the b globin LCR, but only when the transgene was integrated in transcriptionally repressive regions such as found in centromeres (Milot et al., 1996). The subpopulation of erythroid cells that did not transcribe the human b globin gene was associated with a loss of DNase I hypersensitivity within the human b globin locus. Very similar results were obtained in other studies involving partial LCRs (Robertson et al., 1995;Festenstein et al., 1996;Boyer et al., 1997;Guy et al., 1997).Many transcription factors bind to the human b globin locus, but their role in transcriptional activation is not yet clear. An exception is the erythroid-speci®c erythroid Kru Èppel-like factor (EKLF) , which is essential for adult b globin gene expression (Nuez et al., 1995;Perkins et al., 1996;Wijgerde et al., 1996). It binds in vivo to a GT motif with...