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. 2001 Aug 1;15(15):1946–1956. doi: 10.1101/gad.911501

Figure 8.

Figure 8

A model for SAGA functioning as a coactivator for Gal4 by facilitating TATA-binding-protein (TBP) binding to the TATA box of the GAL1 gene. (1) Under noninducing conditions, Gal4 is bound to the UASG via its DNA-binding domain (DB), and the Gal4 activation domain (AD) is blocked by Gal80. (2) After the addition of galactose, the Gal3 inducer is activated and alters the Gal80–Gal4 complex such that the Gal4 activation domain is no longer blocked by Gal80. This change allows the Gal4 activation domain to recruit SAGA to UASG. The presence of the Gal3 protein at the promoter is suggested by the formation of a Gal3–Gal4–Gal80 complex in vitro and in vivo (Chasman and Kornberg 1990; Leuther and Johnston 1992; Parthun and Jaehning 1992; Platt and Reece 1998; Sil et al. 1999). However, other data have suggested that Gal3 is cytoplasmically localized (Peng and Hopper 2000). (3) Once recruited to the promoter, SAGA, mainly via an Spt3-–TBP interaction, recruits TBP to the TATA box to allow transcription initiation.