Proopiomelanocortin (POMC) is expressed predominantly in the corticotrophic cells of the pituitary. Regulatory sequences required for the expression of the human (h) POMC gene were investigated using transient expression of hPOMC-CAT fusion genes in pituitary and nonpituitary cells in combination with DNase I footprint and gel retardation assays. Gene transfer experiments revealed that the hPOMC promoter is more efficiently transcribed in AtT-20 pituitary cells than in HeLa cells. Using deletion analysis, negative regulatory elements between nucleotides -676 and -414 and positive regulatory elements between nucleotides -414 and -93 could be identified. When placed in front of the heterologous thymidine kinase (tk) promoter, nucleotides -414/-223 enhance transcription in AtT-20 cells and in primary cultures of human pituitary tumor cells, but not in various nonpituitary cell lines. In contrast, a -112/-93 element enhances transcription of the tk promoter in all cells tested. DNase I footprint analysis revealed five sites protected by nuclear extracts obtained from AtT-20 cells within nucleotides -414 and -83 of the hPOMC promoter region. In contrast, only one of these sites (between nucleotides -115 and -83) was protected by nuclear extracts from HeLa cells. Gel mobility-shift experiments revealed that an oligonucleotide comprising nucleotides -112/-93 binds a novel nuclear protein. This protein may contribute to the non-cell type-specific expression of the hPOMC gene outside the pituitary, whereas at least five transcription factors seem to be required for high basal transcription of the gene in corticotrophic cells.