LAC REPRESSOR: STRUCTURE AND GENETICS. M. Lewis⁺, M. A. Kercher^*, G. Chang⁺, N. C. Horton^*, J. H. Miller^{[[daggerdbl]]}, H. C. Pace^*, and P. Lu^*. ⁺ Department of Biochemistry and Biophysics and ^*Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA and ^{[[daggerdbl]]}Department of Microbiology and Molecular Genetics, University of California, Los Angeles, CA 90024, USA

The lac operon of E. coli is the paradigm for gene regulation. The key component of the operon is the lac repressor, a product of the lacI gene. The 3-dimensional structures of the intact lac repressor, the lac repressor bound to the gratuitous inducer isopropyl-1-[[beta]]-D-thiogalactoside (IPTG) and the lac repressor complexed with a 21 base-pair symmetric operator DNA have been determined. These three structures show the conformation of the molecule in both the induced and repressed states and provide the framework for understanding a wealth of biochemical and genetic information. A comprehensive site specific substitution analysis on the lac repressor protein has resulted in over four thousand single amino acid replacements of known phenotype between codons 2 through 329 of the lacI gene. We have made an initial correlation of the site-specific mutational analysis with the 3-dimensional structures. In addition, the DNA sequence of the lac operon has three lac repressor recognition sites in a stretch of 500 base pairs. The crystallographic structure of the complex with DNA suggests that the tetrameric repressor functions synergistically with catabolite gene activator protein (CAP) and is involved in the quaternary formation of repression loops in which one tetrameric repressor interacts simultaneously with two sites on the genomic DNA. (supported by NIH and US ARO)