Oct-1 specifically binds the UEF4 Site of the human AP1-regulated urokinase enhancer

The inducible urokinase enhancer contains three essential elements: a combi ned PEA3/AP1 and a downstream AP1 site, separated by a 74-bp DNA region cal led COM (cooperation mediator), that is required for the synergism between the three sites. The 5' half of COM (uCOM) forms four retarded complexes wi th HeLa or HepG2 nuclear proteins (UEF1-4). We now demonstrate that the UEF 4 complex is the transcription factor Oct-1. Because of functional redundan cy of the UEF sites, single mutations in UEF4 have no phenotype; we have ch anged UEF4 from a low to a high affinity binding site for Oct-1. In vitro, this mutation increases the DNA binding of Oct-1 and disturbs the binding o f the Prep-Pbx complexes to the nearby UEF3 site. In vivo, this mutation re duces the basal transcriptional activity of the urokinase enhancer, while n ot affecting its phorbol ester inducibility. This is in keeping with the ef fect of the deletions of the COM region, which result in an increase in the basal level and, as a consequence, in the loss of 4 beta-phorbol 12-myrist ate 13-acetate inducibility. Oct-1 therefore is not involved in the inducib ility of the urokinase enhancer but only in determining its basal activity level.