FUNCTIONAL INTERACTIONS BETWEEN THE ESTROGEN-RECEPTOR AND THE TRANSCRIPTION ACTIVATOR SP1 REGULATE THE ESTROGEN-DEPENDENT TRANSCRIPTIONAL ACTIVITY OF THE VITELLOGENIN A1 IO PROMOTER

Two distinct, TATA box-containing promoters regulate the transcription al activity of the Xenopus vitellogenin Al gene. These two promoters a re of different strength and are separated by 1.8 kilobase pairs of un translated sequence. Estrogen receptor (ER) and its ligand, 17 beta-es tradiol, induce the activity of both promoters. The estrogen response elements (EREs) are located a proximal to the downstream i promoter wh ile no ERE-like sequences have been identified in the vicinity of the upstream io promoter. We show here, that transcriptional activity of t he upstream io promoter is Sp1-dependent. Moreover, we demonstrate tha t estrogen inducibility of the io promoter results from functional int eractions between the io bound Sp1 and the ER bound at the proximity o f i. Functional interactions between Sp1 and ER do not require the pre sence of a TATA box for transcriptional activation, as is demonstrated using the acyl-CoA oxidase promoter. The relative positions that ER a nd Sp1 occupy with respect to the initiation site determines whether t hese two transcription activators can synergize for transcription init iation.