GENETIC-STUDIES OF THE LAC REPRESSOR .10. ANALYSIS OF MISSENSE MUTATIONS IN THE LAC1 GENE

Approximately 2000 non-suppressible mutations in the lacI gene of Escherichia coli have been extensively analyzed. The majority consists of missense mutations resulting in amino acid substitutions in the lac repressor. We characterized each mutation with respect to the resulting altered phenotype, and also mapped them against a large set of deletions. The correlation of the genetic and physical map reported previously has been used to localize the part of the protein affected by each mutation with a high degree of precision (within several amino acids). In particular, we examined the distribution of mutational sites along the gene leading to the i-, is, ir and its phenotypes. Certain regions of the protein, such as the amino-terminal end, are very sensitive to amino acid exchanges with regard to the i- phenotype, whereas other regions are relatively insensitive to substitutions. Of particular interest is the C-terminal half of the gene-protein map, where many Is, and Its mutational sites cluster in very small regions separated by distinct and nearly regularly spaced intervals. The possible significance of these results with respect to repressor structure and function, and to protein structure in general, is discussed. In the following paper we consider the results reported here together with the data from suppressed nonsense mutations, which are described in the preceding paper. © 1979.