Delineation of an evolutionary salvage pathway by compensatory mutations of a defective lysozyme.

Model-free approaches (random mutagenesis, DNA shuffling) in combination with more "rational," three-dimensional information-guided randomization have been used for directed evolution of lysozyme activity in a defective T4 lysozyme mutant. A specialized lysozyme cloning vector phage, derived from phage lambda, depends upon T4 lysozyme function for its ability to form plaques. The substitution W138P in T4 lysozyme totally abolishes its plaque-forming ability. Compensating mutations in W138P T4 lysozyme after sequential random mutagenesis of the whole gene as well as after targeted randomization of residues in the vicinity of Trp138 were selected. In a second stage, these mutations were randomly recombined by the recombinatorial PCR method of DNA shuffling. Shuffled and selected W138P T4 lysozyme variants provide the hybrid lambda phage with sufficient lysozyme activity to produce normal-size plaques, even at elevated temperature (42 degrees C). The individual mutations with the highest compensatory information for W138P repair are the substitutions A146F and A146M, selected after targeted randomization of three residues in the neighborhood of Trp138 by combinatorial mutagenesis. The best evolved W138P T4 lysozymes, however, accumulated mutations originating from both randomly mutagenized as well as target-randomized variants.