LH induction of the progesterone receptor (PR) in granulosa cells is a central event in ovulation. To identify critical regions of the mouse PR promoter that confer LH inducibility in granulosa cells, a mouse PR promoter (-384/+680) genomic fragment was ligated to a luciferase reporter construct and transfected into primary cultures of granulosa cells. Forskolin/phorbol myristate (PMA) induced PR promoter-luciferase reporter activity in granulosa cells greater than 15-fold. A deletion construct comprised only of the distal promoter alone (-348/+64) was inactive. Conversely, deletion constructs eliminating putative distal promoter-regulatory elements that bind Sp1, nuclear factor Y, and GATA-4 as well as the transcription start site (+1) failed to reduce forskolin/PMA activation of reporter activity. Additional 5'-deletions identified a minimal promoter region (+420/+680) sufficient to bestow cAMP responsiveness approximately 8- to 10-fold. Two GC-rich regions Sp1(A)[+440/+461] and Sp1(B) [+473/+490] bound Sp1/Sp3. Site-directed mutagenesis of Sp1(A) and Sp1(B) reduced activity of the proximal (+357/+680) promoter reporter construct approximately 50% and 99%, respectively. When the same Sp1(B) mutation was introduced into the intact promoter (-145/+680), forskolin/PMA induction of promoter activity was reduced by 70-80%. When the distal GC box as well as the proximal Sp1(B) site were both mutated in the context of the intact promoter, inducibility of the transgene was even more severely reduced. The importance of these Sp1/Sp3 binding regions was confirmed in human MCF-7 cells and Drosophila SL2 cells. Collectively, these results indicate that the Sp1/Sp3 binding sites within the mouse PR proximal promoter are essential for transactivation of the gene by agonists in granulosa cells. The molecular mechanisms by which LH activates Sp1/Sp3 at this region within the PR gene remain unknown but do not involve changes in the binding of Sp1/Sp3 to the critical GC boxes. Rather, Sp1/Sp3 appear to recruit other factors to the promoter.