The Subunit Structure of Tryptophan Synthase from Neurospora crassa*

Tryptophan synthase of Neurospora crassa was purified to electrophoretic homogeneity from the wild type strain 74A which had been derepressed by the presence of 0.5 rnr,r indoleacrylic acid in the growth medium. The isolated material migrated as a single symmetrical peak in the ultracentrifuge with a sedimentation constant of 6.0 S. Gel filtration on Sephadex G-ZOO and conventional sedimentation equilibrium yielded molecular weight estimates of 151,000 f 10,000 and 149,000 f 10,000, respectively. Treatment of the enzyme with sodium dodecyl sulfate followed by polyacrylamide gel electrophoresis gave a single band with a relative mobility suggesting a molecular weight of 76,000 f 2000. Aspartic acid was the only detectable NHz-terminal amino acid and experiments with carboxypeptidases A and B revealed that the three amino acids, isoleucine, leucine, and phenylalanine, were released rapidly and in the order mentioned. These results are interpreted as indicating that the Neurosporo enzyme is a homodimer.