FIGURE 9-38 Expression of fusion proteins demonstrates chromatin decondensation in response to an activation domain. A cultured hamster cell line was engineered to contain multiple copies of a tandem array of E. coli lac operator sequences integrated into a chromosome in a region of heterochromatin. (a) When an expression vector for the lac repressor (LacI) was transfected into these cells, lac repressor bound to the lac operator sites could be visualized in a region of condensed chromatin using an antibody against the lac repressor (red). DNA was visualized by staining with DAPI (blue), revealing the nucleus. A diagram of condensed chromatin is shown below. (b) When LacI fused to an activation domain was transfected into these cells, staining as in (a) revealed that the activation domain causes this region of chromatin to decondense into a thinner chromatin fiber that fills a much larger volume of the nucleus. A diagram of a region of decondensed chromatin with bound LacI fusions to the VP16 activation domain (AD) and associated chromatin remodeling and histone acetylase complexes is shown below.