The IE1 and IE2 proteins of human cytomegalovirus transactivate various viral and cellular promoters in a synergistic manner, but the mechanism of their action has not been web elucidated. Here we have examined the IE1-IE2 synergy by artificial recruitment of either Spl or TBP to the promoter. We found that in the presence of Sp1, the synergistic effect of IE1 on IE2-mediated transactivation dramatically decreased. Furthermore, a 117-amino acids glutamine-rich fragment of Spl, which can interact with dTAF(II)110 and hTAF(II)130, was sufficient to replace the role of IE1 in IE1-IE2 synergism. It was also found that TBP recruitment to the promoter markedly decreased the synergistic effect of El on IE2-mediated transactivation. These results suggested that in the context of the synergism between Hi and IE2, the function of IE1 might overlap with that of Spl, for example by recruiting the TFIID complex.