Fig. 1. Transcribed regulatory regions in four loci. (A) Drosophila 87A7 heat-shock locus. Two hsp70 genes (promoters and direction of transcription indicated) are protected from the spread of heterochromatin (grey arrowheads) by scs and scs' (which both contain promoters). (B) Drosophila bithorax complex. Ubx, abd-A and Abd-B are regulated by many silencers and enhancers found in the regions shown below (grey arrows indicate target genes) and various cellular memory modules (CMMs); various barriers (e.g. Mcp, Fab7, Fab8) separate these regions. Not all ORFs, regulators, barriers or transcription units are shown. (C) Yeast HMR locus. A silent domain containing mating type (MAT) loci (a2, a1) is maintained by barriers flanking E and I. Rap1/Abf1/ORC bind to ARS317 (at E) and ARS318 (at I), followed by recruitment of SIR1-4, spread of SIR2-4 to flanking barriers and nucleosomal deacetylation by SIR2. The barrier next to I contains a long terminal repeat (LTR) and a tRNA^Thr gene; the one next to E probably contains one (or two) LTRs, and so also contains promoters. ORF YCR097W-A (downstream of a1) is not shown. (D) Human ß-globin locus. A (transcribed) LCR regulates (grey arrows) the activity of the locus that contains five genes and one pseudogene (ß; not shown). The vertical arrows mark hypersensitive sites (HS).