GAL4 disrupts a repressing nucleosome during activation of GAL1 transcription in vivo.

Photofootprinting in vivo of GAL1 reveals an activation-dependent pattern between the UASG and the TATA box, in a sequence not required for transcriptional activation by GAL4. The pattern results from a nucleosome whose position depends on sequences within the UASG. In the wild-type gene, activation by GAL4 and derivatives disrupts this nucleosome. This activity is independent of interactions with DNA-bound core transcription factors and is proportional to the strength of the activator. Presence of the nucleosome correlates with low basal transcription levels under various conditions, suggesting a role in limiting basal expression. We propose a role for the GAL4 activation domain in displacing a nucleosome and suggest that this is part of the mechanism by which GAL4 activates transcription in vivo.