Interaction of a tissue-specific factor with an essential rat growth hormone gene promoter element

Rat growth hormone (rGH) gene expression is normally restricted to the anterior pituitary. As a model of this tissue specificity, we compared the transient expression of an rGH-chloramphenicol acetyltransferase (CAT) hybrid gene in rGH-producing rat pituitary tumor (GC) cells and in non-rGH-producing rat fibroblast (rat-2) cells. Deletion analysis of the rGH portion of this hybrid gene demonstrated that DNA sequences within 140 base pairs 5' to the rGH gene were sufficient for correct cell type-specific expression. Deletion of an additional 35 base pairs of the rGH 5'-flanking DNA resulted in a loss of expression of the transfected hybrid gene and correlated with the interaction of a putative trans-acting factor with this region of the rGH promoter. This factor was detectable by DNase I footprinting in a crude nuclear extract from GC cells but not from rat-2 cells. Site-directed mutagenesis of the footprint region caused complete loss of expression of a hybrid gene containing 530 base pairs 5' to the rGH gene. Thus, the interaction of this factor, which we term GC2, is likely to be essential for the tissue-specific expression of the rGH gene.