Large-scale chromatin decondensation accompanies gene activation. (A) GFP-lac repressor fluorescence before (top) and after (bottom) gene induction for the DHFR (057L22), Hsp70, and MT BACs. After induction, a more extended “string of dots” is observed. Bar, 2 µm. (B) Histograms showing mean contour lengths (left) or GFP spot number (right) for each BAC cell line at normal growth conditions (CTRL) (DHFR gene induced) and after heat shock (Hsp70 genes induced), Zn addition (MT genes induced), or DMSO induced G1 arrest (DHFR repressed) (see text). Increases in contour length and spot number are specific for the induction conditions appropriate for the particular BAC transgene. (C) Histograms showing contour length under CTRL or induced conditions for Hsp70_5 and MT1_1_4 cell lines with and without transcriptional inhibition. Chromatin decondensation accompanying heat shock is blocked by α-amanitin and significantly reduced by DRB (see text). (B and C) Histogram data points are based on > 100 cells (SEM error bars).