The repressor of the E.coli bacteriophage 434 is similiar in overall structure to lambda repressor and, like lambda repressor, it binds cooperatively to adjacent operator sites on DNA. Unlike lambda repressor, however, 434 repressor does not bind cooperatively to separated sites on DNA as assayed in vitro. The objective of this proposal is to investigate the basis of this difference in an array of experiments. Genetically engineered 434-lambda repressor hybrids will be generated and studied in order to determine the basis of the observed differences in the abilities of 434 and lambda repressor to bind cooperatively to nonadjacent operator sites. A system for studying cooperatively "at a distance" in vivo using 434 operators will be established. This system will be used to detect and investigate hybrid and mutant repressors. Efforts will be made to ascertain whether specific "DNA-bending" sequences in the OL region of the phage allow 434 repressor to bind cooperatively to nonadjacent sites. These studies should provide new insight into those aspects of protein structure necessary for interaction of DNA-bound regulatory proteins "at a distance".