DOMAINS OF TRANSCRIPTION FACTOR SP1 REQUIRED FOR SYNERGISTIC ACTIVATION WITH STEROL REGULATORY ELEMENT-BINDING PROTEIN-1 OF LOW-DENSITY-LIPOPROTEIN RECEPTOR PROMOTER

Feedback regulation of transcription from the low density lipoprotein (LDL) receptor gene is fundamentally important in the maintenance of i ntracellular sterol balance, The region of the LDL receptor promoter r esponsible for normal sterol regulation contains adjacent binding site s for the ubiquitous transcription factor Sp1 and the cholesterol-sens itive sterol regulatory element-binding proteins (SREBPs), Interesting ly, both are essential for normal sterol-mediated regulation of the pr omoter, The cooperation by Sp1 and SREBP-1 occurs at two steps in the activation process, SREBP-1 stimulates the binding of Sp1 to its adjac ent recognition site in the promoter followed by enhanced stimulation of transcription after both proteins are bound to DNA. In the present report, we have defined the protein domains of Sp1 that are required f or both synergistic DNA binding and transcriptional activation, The ma jor activation domains of Sp1 that have previously been shown to be es sential to activation of promoters containing multiple Sp1 sites are r equired for activation of the LDL receptor promoter, Additionally, the C domain is also crucial. This slightly acidic approximate to 120-ami no acid region is not required for efficient synergistic activation by multiple Sp1 sites or in combination with other recently characterize d transcriptional regulators, We also show that Sp1 domain C is essent ial for full, enhanced DNA binding by SREBP-1. Taken together with oth er recent studies on the role of Sp1 in promoter activation, the curre nt experiments suggest a unique combinatorial mechanism for promoter a ctivation by two distinct transcription factors that are both essentia l to intracellular cholesterol homeostasis.