(2'-5')Oligoadenylate [(2'-5')(A)n] synthetase is a key enzyme in the interferon-elicited antiviral response whose controlled expression in interferon-treated cells has been only partially elucidated. In this investigation, we have compared the modulation of the (2'-5')(A)n synthetase gene by interferon alone and by the combination of interferon and a second cellular effector, 12-O-tetradecanoyl-phorbol 13-acetate (TPA). Although TPA alone had no effect on (2'-5')(A)n synthetase, it potentiated the induction of (2'-5')(A)n of synthetase by interferon in HL-60 and HeLa cells by increasing content of its mRNA and an immunoreactive 40-kDa isoenzyme. Since TPA activates protein kinase C (PKC), other PKC-activating phorbol-ester analogues were tested and found to be effective, whereas the PKC inhibitor staurosporine reduced the potentiative activity of TPA. By using the (2'-5')(A)n synthetase gene promoter linked to a reporter gene, chloramphenicol acetyltransferase (CAT), TPA and interferon were found to result in a doubling of CAT activity compared to cells treated with interferon alone. Moreover, when nuclear extracts prepared from control cells or cells treated with TPA and interferon (IFN), separately or together, were incubated with radioactively labeled oligodeoxynucleotides containing the interferon-responsive element (IRE), TPA was shown to down-regulate an IFN-inducible IRE/protein complex. These data further suggest that TPA regulates (2'-5')(A)n synthetase gene expression at the level of transcription.