The Formation and Properties of Dimers of the Tryptophan Synthetase a Subunit of Escherichia coZi*

The normally monomeric 01 subunit of Escherichia coti tryptophan synthetase forms dimers and higher order aggregates following exposure to high urea concentrations and removal of the urea by dialysis. The maximum yield of cx chain dimers is approximately 2%. Dimers of certain combinations of different enzymatically inactive mutant (Y monomers exhibit the missing enzymatic activity. In such heterologous dimers, the reconstituted active site has at least 50% of the activity of the active site of the native wild type 01 monomer. a! chain dimers form an enzymatically active complex with the tryptophan synthetase & subunit. However, an (Y chain dimer can bind only a single /32 molecule. The cx chain dimer dissociates into CY monomers when heated at 51”. A tentative model of the structure of an 01 chain dimer is presented.