Transcription factor dosage affects changes in higher order chromatin structure associated with activation of a heterochromatic gene

The mechanisms of transcriptional activation in heterochromatin were invest igated by using FISH to directly visualize changes in chromatin organizatio n during activation of a heterochromatic lambda5 transgene. A DNase I hyper sensitive site was shown to relocate the transgene to the outside of the pe ricentromeric heterochromatin complex in the absence of transcription. Acti vation of transcription, which is dependent on the transcription factor EBF , occurs in a stochastic manner that resembles telomeric silencing in yeast , with the transcribed gene remaining closely associated with the heterochr omatin complex. Reducing the dosage of EBF results in a reduced frequency o f localization of the transgene to the outside of the heterochromatin compl ex and lower levels of transcription. These data provide evidence that tran scription factors can initiate changes in higher order chromatin structure during the earliest stages of gene activation.