We are investigating the role of the early response transcription factor, nerve growth factor inducible A gene (NGFI-A), as a modulator of retinoblastoma (RB) gene transcription in prostate cells. Examination of the RB promoter reveals a novel element GCGGGGGAG located at nucleotides 152-144 upstream of the methionine initiation codon. This sequence shares strong homology with the consensus NGFI-A binding element GCGGGGGCG varying by a single nucleotide. In DNA binding assays, an NGFI-A fusion protein and the native protein product of the NGFI-A gene purified from prostate cancer cells bound specifically to an oligonucleotide containing the RB promoter element. Gene expression studies in rat ventral prostate demonstrated a 1.9-fold increase in RB mRNA following castration that parallels a 2.7-fold induction of NGFI-A mRNA. In summary, the in vitro DNA binding data and the transient coregulation of rat NGFI-A and RB following castration suggests that the RB gene may be transcriptionally regulated by NGFI-A in prostate cells.