The association of poised genes with transcription factories may contribute to rapid transcriptional activation in response to stimuli and to silencing when genes are located at the interior of their chromosome territories.

We show the interaction between the enhancer and the minimal promoter of urokinase-type plasminogen activator gene during active transcription by coupling micrococcal nuclease digestion of cross-linked, sonicated chromatin, and chromatin immunoprecipitation. This approach allowed the precise identification of the interacting genomic fragments, one of which is resistant to micrococcal nuclease cleavage. The interacting fragments form a single transcriptional control unit, as indicated by their common protein content. Furthermore, we show that the enhancer-MP interaction persists during the early stages of transcription and is lost upon ␣-amanitin treatment, indicating the requirement for active transcription. Our results support a looping model of interaction between the enhancer and the MP of the urokinase-type plasminogen activator gene.Transcription regulation in eukaryotic cells is a multistep process that involves the assembly of multiprotein complexes on gene regulatory regions (1). These regulatory regions contain two types of sequences: enhancers/silencers, which recruit a complex array of transcription factors and chromatin-modifying activities, and core promoter elements to which the general transcriptional machinery, including RNAP II, 2 is recruited. Understanding the molecular mechanism(s) involved in transcriptional regulation over long distances is of fundamental importance, because in most cases the activities of remote control elements are essential in turning on or off specific subsets of genes in a temporally and spatially regulated manner (2). The main models to explain distal enhancer function invoke enhancer-promoter communication, either through proteinprotein interactions resulting in the formation of DNA/chromatin loops (looping model), the free sliding of proteins recruited on the enhancer along the DNA (scanning model), or the establishment of modified chromatin domains between the enhancer and the promoter by facilitator proteins, which generate a progressive chain of higher order complexes along the chromatin fiber (3-9). The looping model, however, is supported by recent results in a number of different experimental systems, showing that the close physical proximity of distant regulatory sequences is required for proper gene expression (10 -15). The interaction between enhancer elements and proximal promoter culminates in the transcription activation event. However it is not yet clarified if the activation of transcription stems from the transient interaction of regulatory elements or through a more stable structure working as a single control unit.The uPA gene codes for a serine protease involved in the degradation of the extracellular matrix and in cell motility (16). The expression of this gene is normally dependent on the presence of inducers, but in cancer cells it is often constitutive. The regulatory region of the uPA gene contains an MP and an enhancer located 2 kb upstream (17). In highly invasive human prostate adenocarcinoma (PC3) cells this gene is constitutively expresse...