ALTERATION OF T4 LYSOZYME STRUCTURE BY 2ND-SITE REVERSION OF DELETERIOUS MUTATIONS

Mutations that suppress the defects introduced into T4 lysozyme by sin gle amino acid substitutions were isolated and characterized. Among 53 primary sites surveyed, 8 yielded second-site revertants; a total of 18 different mutants were obtained. Most of the restorative mutations exerted global effects, generally increasing lysozyme function in a nu mber of primary mutant contexts. Six of them were more specific, suppr essing only certain specific deleterious primary substitutions, or dim inishing the function of lysozymes bearing otherwise nondeleterious pr imary substitutions. Some variants of proteins bearing primary substit utions at the positions of Asp 20 and Ala 98 are inferred to have sign ificantly altered structures.