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The Drosophila HMG1-like protein DSP1 was identified by its ability to inhibit the transcriptional activating function of Dorsal in a promoter-specific fashion in yeast. We show here that DSP1 as well as its mammalian homolog hHMG2 bind to the mammalian protein SP100B and that SP100B in turn binds to human homologs of HP1. The latter is a Drosophila protein that is involved in transcriptional silencing. Each of these proteins represses transcription when tethered to DNA in mammalian cells. These results suggest how heterochromatin proteins might be recruited to specific sites on DNA with resultant specific effects on gene expression.The Drosophila protein Dorsal can act as a transcriptional activator or repressor depending on the promoter context. For example, in Drosophila, Dorsal activates the twist promoter but represses the zen promoter. A Dorsal-binding element taken from the zen promoter, called the ventral repression element, and placed upstream of an activated gene in a Drosophila embryo, mediates Dorsal-dependent repression of that gene (1). In Saccharomyces cerevisiae, however, Dorsal activates transcription from both the twist and zen promoters. DSP1, a member of the high mobility group1͞2 (HMG) family of non-histone chromosomal DNA-binding proteins, was isolated as a putative corepressor that inhibits Dorsal from activating the zen promoter but has no effect on Dorsal activation of a reporter bearing certain isolated Dorsal-binding sites (2). DSP1 interacts with Dorsal and with p50͞p65 heterodimer NF-B and binds cooperatively with these proteins to DNA (ref. 2 and J. Brickman and M.P., unpublished data).

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The split-ubiquitin assay detects protein interactions in vivo. To identify proteins interacting with Gal4p and Tup1p, two transcriptional regulators, we converted the split-ubiquitin assay into a generally applicable screen for binding partners of specific proteins in vivo. A library of genomic Saccharomyces cerevisiae DNA fragments fused to the N-terminal half of ubiquitin was constructed and transformed into yeast strains carrying either Gal4p or Tup1p as a bait. Both proteins were C-terminally extended by the C-terminal half of ubiquitin followed by a modified Ura3p with an arginine in position 1, a destabilizing residue in the N-end rule pathway. The bait fusion protein alone is stable and enzymatically active. However, upon interaction with its prey, a native-like ubiquitin is reconstituted. RUra3p is then cleaved off by the ubiquitin-specific proteases and rapidly degraded by the N-end rule pathway. In both screens, Nhp6B was identified as a protein in close proximity to Gal4p as well as to Tup1p. Direct interaction between either protein and Nhp6B was confirmed by coprecipitation assays. Genetic analysis revealed that Nhp6B, a member of the HMG1 family of DNA-binding proteins, can influence transcriptional activation as well as repression at a specific locus in the chromosome of the yeast S. cerevisiae. The split-ubiquitin method is based on the ability of N ub and C ub , the N-and C-terminal halves of ubiquitin, to form a native-like ubiquitin (1). Ubiquitin-specific proteases (UBPs), present in the cytosol and nucleus of all eukaryotic cells, recognize the reconstituted ubiquitin, but not its halves, and cleave off a reporter protein, which had been linked to the C terminus of C ub . The split-ubiquitin assay (split-Ub) is designed to yield efficient association of N ub and C ub only if the two ubiquitin halves are linked to proteins that interact in vivo. The assay has been shown to detect interactions between cytosolic proteins, membrane proteins, and transient interactions that occur between transporter and substrate during protein translocation across the membrane of the endoplasmic reticulum in vivo (1-4). In addition, split-Ub can also be used to demonstrate interactions between transcription factors (5, 6) because, contrary to the two-hybrid system (7), it is not based on a transcriptional readout.The Saccharomyces cerevisiae GAL1 promoter is a wellstudied example of transcriptional regulation by nutrients. When the cells are grown in medium containing galactose as the sole carbon source, GAL1 is activated by Gal4p, which binds specifically to the GAL1 promoter. Gal4p interacts with the holoenzyme component Srb4p, thereby recruiting the transcription apparatus to the GAL1 promoter (8). If the carbon source is switched to glucose, the promoter is repressed by two independently operating mechanisms. Gal80p masks the activation domain of DNA-bound Gal4p, thereby preventing the recruitment of the transcription machinery (9). In addition, the cytosolic repressor Mig1p enters the nucleus (10). Mig1p b...

In recent years, the exposure of human skin to environmental and artificial UV irradiation has increased dramatically. This is due not only to increased solar UV irradiation as a consequence of stratospheric ozone depletion, but also to inappropriate social behaviour with the use of tanning salons still being very popular in the public view. Besides this, leisure activities and a lifestyle that often includes travel to equatorial regions add to the individual annual UV load. In addition to the common long-term detrimental effects such as immunosuppression and skin cancer, the photo-oxidative damage due to energy absorption of UV photons in an oxygenized environment leads to quantitative and qualitative alterations of cells and structural macromolecules of the dermal connective tissue responsible for tensile strength, resilience and stability of the skin. The clinical manifestations of UV/reactive oxygen species (ROS)-induced disturbances result in photoaged skin with wrinkle formation, laxity, leathery appearance as well as fragility, impaired wound healing capacities and higher vulnerability. Strategies to prevent or at least minimize ROS-induced photo-ageing and intrinsic ageing of the skin necessarily include protection against UV irradiation and antioxidant homeostasis.