TRYPTOPHAN SYNTHASE FROM SACCHAROMYCES-CEREVISIAE IS A DIMER OF 2 POLYPEPTIDE-CHAINS OF MR 76000 EACH

Tryptophan synthase of S. cerevisiae was purified from a derepressed mutant to homogeneity. The crucial step in the new procedure is hydrophobic interaction chromatography on pentyl-Sepharose in strong (NH4)2SO4 solutions. Tryptophan-synthase-inactivating agents are efficiently removed by this step. The protein migrates as a single boundary in the ultracentrifuge with a sedimentation coefficient of 6.1 S. Its MW as judged by sedimentation equilibrium measurements is 167,000. Treatment of the enzyme with sodium dodecyl sulfate followed by gel electrophoresis in polyacrylamide reveals a single band with an apparent MW of 76,000. Gel filtration studies with Sephacryl S-300, performed under conditions that suppress unspecific absorption of proteins, indicate a MW of 390,000. Tryptophan synthase from Escherichia coli, having a MW of 148,000, is also eluted at a position corresponding to an apparently higher MW of 280,000. These results are interpreted in the context of other genetic and biochemical information on tryptophan synthase from S. cerevisiae, Neurospora crassa and E. coli as signifying that the yeast enzyme is a dimer of bifunctional polypeptide chains. Previously, tryptophan synthase from S. cerevisiae was regarded as an oligomer of 4 polypeptide chains of MW 37,000 each. It is probably the product of inadvertent proteolysis.