A COOPERATIVE INTERACTION BETWEEN NF-KAPPA-B AND SP1 IS REQUIRED FOR HIV-1 ENHANCER ACTIVATION

The human immunodeficiency virus (HIV-1) long terminal repeat (LTR) co ntains two binding sites for NF-chiB in close proximity to three bindi ng sites for the constitutive transcription factor, Spl. Previously, s timulation of the HIV enhancer in response to mitogens has been attrib uted to the binding of NF-chiB to the viral enhancer. In this report, we show that the binding of NF-chiB is not by itself sufficient to ind uce HIV gene expression. Instead, a protein - protein interaction must occur between NF-chiB and Sp1 bound to an adjacent site. Cooperativit y both in DNA binding and in transcriptional activation of NF-chiB and Sp1 was confirmed by electrophoretic mobility shift gel analysis, DNa se footprinting, chemical cross-linking and transfection studies in vi vo. With a heterologous promoter, we find that the interaction of NF-c hiB with Sp1 is dependent on orientation and position, and is not obse rved with other elements, including GATA, CCAAT or octamer. An increas e in the spacing between the chiB and Sp1 elements virtually abolishes this functional interaction, which is not restored when these sites a re brought back into the same helical position. Several other promoter s regulated by NF-chiB also contain chiB in proximity to Spl binding s ites. These findings suggest that an interaction between NF-chiB and S p1 is required for inducible HIV-1 gene expression and may serve as a regulatory mechanism to activate specific viral and cellular genes.