Genetic studies of the Lac repressor. XV: 4000 single amino acid substitutions and analysis of the resulting phenotypes on the basis of the protein structure.

Each amino acid from position 2 to 329 of Lac repressor was replaced by 12 or 13 of the 20 natural occurring amino acids. The resulting phenotypes are discussed on the basis of (1) the recently published structure of the Lac repressor core complexed with the inducer IPTG and (2) a model of the dimeric Lac repressor built by homology modelling from the X-ray structure of the purine repressor-corepressor-operator complex. This phenotype analysis, based on 4000 well-defined mutants, yields a functional description of each amino acid position of Lac repressor. In most cases, mutant effects can be directly correlated with the structure and function of the protein. This connection between the amino acid position and the structure and function of the protein is in most cases direct and not complicated: amino acids which are directly involved in sugar binding are affected in Lac repressor mutants of the Is type; small amino acids which can only be replaced by other small acids are located in the core of the protein; positions at which nearly all amino acids are tolerated are in most cases located on the surface of the protein. Amino acids which are highly conserved throughout the LacI family of repressors, and not directly involved in specific functions of the protein like DNA recognition or sugar binding, form a network of contacts with other amino acids. Such amino acids are either located inside one subunit, mostly at the interface between secondary structure elements, or are involved in the dimerisation interface.