GENETIC-STUDIES OF THE LAC REPRESSOR .9. GENERATION OF ALTERED PROTEINS BY THE SUPPRESSION OF NONSENSE MUTATIONS

We have generated more than 300 altered lac repressor proteins carrying known amino acid replacements, by employing nonsense mutations at 90 positions in the lacI gene together with eight different nonsense suppressors. This allows the substitution of lysine, serine, tyrosine, leucine and glutamine at virtually all of the respective positions in the repressor, and tryptophan at ten positions in the repressor. Since most of the nonsense sites have been correlated with specific codons in the lacI messenger RNA, in almost all cases the position of the substituted residue is known. This process results in the creation of a large number of mutant phenotypes. We have analyzed the effects of each substitution in vivo, and in several cases studied partially purified repressors in vitro. The properties of the altered proteins have been compared with the position and nature of each exchanged residue. We discuss the implications of these findings with regard to repressor structure in particular, and to protein structure in general. Further applications of the suppression method are also considered. © 1979.