UASrpg can function as a heterochromatin boundary element in yeast

The HM loci in Saccharomyces cerevisiae constitute region-specific but gene-nonspecific repression domains, as a number of heterologous genes transcribed by RNA polymerase II or III are silenced when placed at these loci. The promoters of the Ashbya gossypii TEF gene and the S. cerevisiae TEM and TEF2 genes, however, are resistant to transcriptional silencing by the HM silencers in yeast. Moreover, when interposed between the HML alpha genes and the E silencer, certain segments of these promoters block the repression effect of the silencer on the alpha genes. All of these fragments contain UAS(rpg) (upstream activation sequence of ribosome protein genes) composed of multiple binding sites for Rap1. In fact, a 149-bp segment consisting essentially of only three tandem Rap1-binding sites from the UAS(rpg) of yeast TEF2 exhibits silencer-blocking activity. This element also exhibits insulating activity and orientation dependence characteristic of known chromatin boundary elements. finally, the element blocks the physical spread of heterochromatin initiated at a silencer. This segment provides the first example of chromatin domain boundary or insulator elements in yeast.