Practice question

Gal4 contains Zn2C6 binuclear cluster DNA-binding domain recognizing UASG as dimer plus activation domains. Without galactose, Gal80 repressor binds Gal4 activation domain region 851-874, occluding surfaces for Mediator subunit Med15, SAGA acetyltransferase, and TFIIH recruitment. Masking retains Gal4 on DNA but creates inert complex unable to stimulate initiation. Gal80 does not evict Gal4, only blocks polymerase II holoenzyme recruitment. Upon galactose sensing, Gal3 sequesters Gal80, freeing activation domain to drive multiple transcription rounds and chromatin remodeling at GAL promoters enabling galactose catabolism gene expression strongly.