Restoration of Enzymic Activity by Complementation in Vitro between Mutant α Subunits of Tryptophan Synthetase and between Mutant Subunits and Fragments of the α Subunit

The wild type and all mutant α chains of tryptophan synthetase tested can, following denaturation-renaturation, enter into stable dimeric structures. Only certain combinations of mutant α chains yield dimers which have the enzymatic activity which the mutant monomers lack. Most α chains with mutational alterations in the NH2-terminal half of the molecule will complement most α chains that are altered in the COOH-terminal half. One of the five fragments generated by cyanogen bromide cleavage of the α chain complements (restores activity to) an intact mutant α chain. The site of the amino acid change in the mutant subunit is overlapped by the sequence in the fragment. Some fragments of the α chain generated by the presence of a nonsense (ochre) codon in the structural gene for the α chain are able to complement appropriate intact mutant α chains. The relative size of two such fragments is consistent with the map position of the nonsense codons. The presence of large amounts of other Escherichia coli proteins in the urea solution from which α chain dimers are recovered has only a small effect on the ability of α chains to renature as monomers. Similarly, there is little effect of nonspecific protein on the ability of mutant α chains to form functional dimers. In contrast, the β2 subunit of tryptophan synthetase appears to have a specific effect upon both α chain dimer formation and renaturation of the α monomer.