Wehave~ fousfymportedthefnesenceotahonnoneinducible transcriptional activation function (TAPP) within the region of the estrogen receptor (ER) that contains the hormone binding domain. We show here that the N-terminal A/B region of the ER contains an indemndent constftutive activation tunction (TAM) that exhibits ceil typa specificfty since it activates transcription efficiently in chicken embryo fibrobfasts, but only poorly in HeLa ceils. By analyzing the ability of TAM, TAF2, and the GAL4 and VP16 acidk activating domains (AADa) to homosynergize and heterosynegixe with one another and with the factor binding to the uprtmam element (UE) of the adenovirus 2 major fate promoter, we show that the activation properties of TAM and TAM are dftterent and distinct from those ot AAUs, in agreement with ths &sence ot acidic amino acid stretches in TAP1 and TAP2. introduction

The nuclear receptors represent a family of iigand-inducible transcriptional enhancer factors (Green and Chambon, 1988; Evans, 1988). in the presence of figand they activate transcription through recognition of cognate responsive elements@ Es). Amino acid sequence comparisons have shown that these receptors are composed of a series of conserved domains (Krust et al., 1986). The DNA binding domain (DBD)(region C), the most highly conserved among the receptors, is a 66-68 amino acid long region containing two putative zinc fingers (Evans and Hoflenberg, 1988; Green and Chambon, 1988; Mader et al., 1989; and references therein). The hormone binding domains (HBD)(region E) are fess well conserved among the receptors. Using chimeric molecules composed of a HBD finked to the DBD of the yeast transcription activator GAL4, the HBD of the estrogen and glucocorticoid receptors were shown to contain figand-inducible transcription activation functions (Webster et al., 1988b). Recent experiments have demonstrated that the activation function located in the HBD of the human estrogen receptor (ER)