The transcription factors Sp1, Sp3, and Egr-1 bind with their zinc finger DNA-binding domains to GC-rich sequences in the regulatory regions of their target genes. The similarity of the DNA-binding sites of Sp1, Sp3, and Egr-1 has triggered the hypothesis that they compete for the same DNA-binding site. We have investigated the specificity of transcriptional regulation by Sp1, Sp3, and Egr-1 using dominant-negative mutants that block the DNA-binding site of Sp1, Sp3, or Egr-1, respectively. The results show that constitutive transcription of Sp1 regulated reporter genes, containing Sp1 sites derived from the aldolase C and p21WAF1/Cip1 genes, or the long terminal repeat of HIV-1, was impaired by dominant-negative mutants of Sp1 and Sp3, but not by a dominant-negative Egr-1. Transcription mediated by Egr-1 was induced by transfection of expression vectors encoding wild-type or mutated Egr-1 or by stimulation of the extracellular signal-regulated protein kinase pathway via an inducible B-Raf-estrogen receptor fusion protein. In all cases transcription of Egr-1-regulated reporter genes, containing Egr-1 binding sites derived from the Egr-1 or the synapsin I gene was impaired by a dominant-negative Egr-1, but not by dominant-negative Sp1 or Sp3 mutants. These results show that there are genuine Sp1/Sp3 or Egr-1 controlled genes showing no cross-regulation of Sp1/Sp3 and Egr-1 through the same DNA-binding site. This does not exclude the existence of composite Sp1/Sp3/Egr-1 binding sites, where competition for a common DNA-binding site occurs. © 2004 Wiley-Liss, Inc.