Fig. 2. lacI-HP1 association renders the hsp26 promoter less accessible. (A) The hsp26 reporter gene (not to scale) contains a TATA box, HSEs and (GA)_n elements within two DNaseI hypersensitive regions. XbaI sites within the HSEs were used for restriction enzyme accessibility. (B) Accessibility of the hsp26 promoter region was determined under non-tethering, GFP-tethering and HP1-tethering conditions. Nuclei were isolated and treated with an excess of XbaI. The DNA was purified, digested to completion with SalI, and analyzed by Southern analysis using the unique hsp26-tag sequences for hybridization. The percent accessibility is shown below each lane. (C) MNase accessibility of the hsp26 promoter was determined in homozygous larvae containing either the lac-hsp26-hsp70 reporter and the lacI-HP1 expressor transgene, or the reporter gene alone. Nuclei isolated from homozygous third instar larvae were treated with increasing amounts of MNase. The DNA was purified and assayed by Southern analysis using the unique hsp26 tag sequences for hybridization. The left pair of membranes was re-hybridized with heterochromatic sequences upstream of the rolled locus (shown on right). A densitometry trace through the fourth lane (top to bottom) of each membrane is plotted below.