Genetic and Enzymatic Experiments relating to the Quaternary Structure of ß-Galactosidase

The quaternary structure of f:!-galactosidase, which consists of four identical subunits, has been studied by the isolation and characterization of appropriate mutants of Escherichia coli. Of 146 mutants examined, 19 were found to have enzymes with reduced subunit association. These altered enzymes are especially sensitive to inactivation by urea which, at concentrations that do not affect the normal enzyme, causes dissociation into subunits. Mapping with overlapping deletions showed that the mutations affecting quaternary structure are not distributed continuously, but occur in five or six groups within the gene. The mapping indicates that polypeptide sequences involved in subunit association and in substrate binding are contiguous. A model for f:!-galactosidase structure is suggested in which substrate binding sites are provided by the clefts formed between subunits when they associate.