We have studied the effects of chromomycin and of a triple-helix-forming oligonucleotide (TFO) that recognizes Sp1 binding sites on protein-DNA interactions and HIV-1 transcription. Molecular interactions between chromomycin, the Sp1 TFO and target DNA sequences were studied by gel retardation, triplex affinity capture using streptavidin-coated magnetic beads and biosensor technology. We also determined whether chromomycin and a TFO recognizing the Sp1 binding sites of the HIV-1 long terminal repeat (LTR) inhibit the activity of restriction enzyme HaeIII, which recognizes a sequence (5'-GGCC-3') located within these Sp1 binding sites. The effects of chromomycin and the TFO on the interaction between nuclear proteins or purified Sp1 and a double-stranded oligonucleotide containing the Sp1 binding sites of the HIV-1 LTR were studied by gel retardation. The effects of both chromomycin and TFO on transcription were studied by using an HIV-1 LTR-directed in vitro transcription system. Our results indicate that low concentrations of chromomycin potentiate the effects of the Sp1 TFO in inhibiting protein-DNA interactions and HIV-1-LTR-directed transcription. In addition, low concentrations of chromomycin do not affect binding of the TFO to target DNA molecules. The results presented here support the hypothesis that both DNA binding drugs and TFOs can be considered as sequence-selective modifiers of DNA-protein interactions, possibly leading to specific alterations of biological functions. In particular, the combined use of chromomycin and TFOs recognizing Sp1 binding sites could be employed in order to abolish the biological functions of promoters (such as the HIV-1 LTR) whose activity is potentiated by interactions with the promoter-specific transcription factor Sp1.