Biological reagents that selectively regulate the expression of oncogenes, tumor suppressors, or other cancer-related genes would be powerful tools for research and for therapeutics. The key to developing such reagents is to identify molecules capable of sequence-specific recognition of DNA. We have recently devised a strategy based on combinatorial peptide libraries expressed in yeast that allows the rapid selection of sequence-specific DNA binding peptides. We have demonstrated that peptides identified by this means can substitute for a zinc-finger DNA binding domain in a chimeric transcription factor. We now intend to further develop our yeast combinatorial library strategy, and to use this strategy to create molecules able to selectively regulate cancer-related genes. In Specific Aim I we will attempt to block the expression of the drug resistance related MDR1 gene as a model for selective gene inhibition in mammalian cells. This will be done by identifying peptides that specifically bind to the MDR1 promoter with high affinity and that block access to a key transcription factor binding site. In Specific Aim II we will attempt to selectively activate expression of sub-sets of genes regulated by the p53 tumor suppressor protein. In particular, we will focus on p21, a gene involved in cell cycle arrest, and bax, a gene involved in apoptosis. We will create chimeric transcription factors able to bind selectively either to a p53 site in the p21 promoter, or to a distinct p53 site in the bax promoter. We will test whether the chimeric transcription facts can selectively increase expression of p21 or Bax, and we will evaluate the impact on cell cycle arrest or apoptosis. In Specific Aim III we will explore several approaches for improving the yeast combinatorial library strategy. As improvements are found they will be utilized in the pursuit of the first two specific aims. We believe that the investigations proposed here will provide important insights into fundamental aspects of protein-DNA interactions. We also believe that they will create opportunities for the development of novel approaches to cancer therapy.