GENETIC SELECTION FOR MUTATIONS THAT IMPAIR THE COOPERATIVE BINDING OF LAMBDA REPRESSOR

Bacteriophage lambda repressor binds co-operatively to adjacent pairs of DNA target sites. A novel combination of positive genetic selection s, involving two different operon fusions derived from P22 challenge p hages, was used to isolate mutant lambda repressors that have lost the ability to bind co-operatively to tandem sites yet retain the ability to bind a strong, single site. These cb (co-operative binding) mutati ons result in 10 different amino acid changes, which define eight resi dues in the carboxyl-terminus of repressor. Because challenge phage de rivatives may be applied to study essentially any specific protein-DNA interaction, analogous combinations of genetic selections may be used to explore the ways that a variety of proteins interact to assemble r egulatory complexes.