We have previously shown that the human immunodeficiency virus type 1 (HIV-1) Tat protein can activate a synthetic promoter containing consensus-binding sites for the cellular transcription factor Sp1. In this report, we show that a GAL-Tat fusion protein targeted via GAL4 DNA-binding sites can also trans activate an HIV-1 LTR promoter independently of the trans-activation response region. To show that the trans activation of the promoter by Tat directly involves the Sp1 protein, we have targeted a GAL-Sp1 fusion protein to the long terminal repeat promoter via upstream GAL4-binding sites. In the presence of Tat and GAL-Sp1, the promoter is synergistically trans activated at the transcriptional level, indicating that Tat and Sp1 functionally interact to trans activate the HIV-1 promoter. The Sp1 synergism is relatively specific, since another chimeric transcriptional activator, GAL-VP16, does not appear to be significantly synergistic with Tat.