We have previously identified poly(ADP-ribose) polymerase-1 (PARP-1) as a coactivator for the human T cell leukemia virus type I (HTLV-I) transcription activator Tax. While PARP-1 is believed to contribute to DNA repair, PARP-1 has been described as a coactivator for other transcription factors. Recent evidence suggests that PARP-1 forms complexes on cellular promoters, so we investigated PARP-1 complexes on the HTLV-I Tax responsive elements (TxREs) using an end-blocked DNA binding assay. We observed sequence-specific binding of PARP-1 to the TxREs. The DNA binding domain of PARP-1 was fused to the transcriptional activation domain of VP16, and this fusion protein activated the HTLV-I promoter in a TxRE-dependent manner. Internal, sequence-specific binding of PARP-1 to DNA provides a mechanism for transcriptional regulation of the HTLV-I promoter. The mechanism of PARP-1 function in the HTLV-I system may have common mechanistic steps with other cellular promoters, including the formation of active complexes on the promoter.