We have determined the structural organization of pregnancy-specific glycoprotein subclass 1 (PSG1-I) and subclass 3 (PSG12) genes and identified the cis-acting elements in the PSG 5'-flanking regions required for expression. Nucleotides -835/-34 (PSG12) or -834/-34 (PSG1- I) upstream of the translational start site contain both positive and negative regulatory elements that modulate expression in human placental cells. The -172 to -74 DNA region contains the PSG12 promoter elements including a binding site(s) for protein factors in the human placental cell extract. We have immortalized human placental cell lines with adenovirus-origin- minus SV40 recombinant viruses containing wither wild-type (wt) or temperature-sensitive (ts) A mutants of SV40. Cells transformed with the SV40 tsA chimera (HP-A1 and HP-A2), but not the SV40 wt chimera (HP-W1), were ts for transformation. All three cell lines express the trophoblast-specific genes PSG and the alpha and beta subunits of hCG and support expression of PSG promoter-CAT fusion gene. We have also examined the molecular mechanisms that activate germ cell alkaline phosphatase AP (GCAP) but inactivate placental AP (PLAP) expression in human choriocarcinoma cells. Different cis-acting elements in the 5' -flanking regions of GCAP and PLAP genes control their differential expression. Choriocarcinoma-specific expression is mediated within the -156 to -1 bp region relative to the GCAP transcription start site (+1). Within the minimal enhancer element, three sites, I (-63/- 44), II (-87/-67), and III (-136/-103), were shown to bind choriocarcinoma nuclear proteins. Since the 5'-flanking region in the PLAP promoter lacks the high affinity protein binding site I and II, choriocarcinoma cells express low or undetectable levels of the PLAP gene.