The transcription complex activator protein-1 (AP-1) plays a role in a diverse number of cellular processes including proliferation, differentiation, and apoptosis. To identify AP-1-responsive target genes, we used a doxycycline-inducible c-Jun system in Rat1a cells. The HMG-I/Y chromatin binding protein was found to be up-regulated by c-Jun. Following induction of c-Jun expression, Rat1a cells under nonadherent growth conditions have sustained HMG-I/Y mRNA expression and 2-fold higher protein than uninduced cells. HMG-I/Y promoter reporter assays show that HMG-I/Y promoter activity increases in the presence of c-Jun expression, and gel mobility shift assays demonstrate that induced c-Jun binds to an AP-1 consensus site at position -1,091 in the HMG-I/Y promoter. Suppression of HMG-I/Y expression by its antisense sequence significantly reduces the ability of c-Jun-overexpressing Rat1a cells to grow in an anchorage-independent fashion. HMG-I/Y transforms Rat1a cells (although the colonies are smaller than that observed for the cells overexpressing c-Jun). Taken together, these results suggest that HMG-I/Y is a direct transcriptional target of c-Jun necessary for c-Jun-induced anchorage-independent growth in Rat1a cells.