A dimer of a single polypeptide chain catalyzes the terminal four reactions of the L-tryptophan pathway in Euglena gracilis.

In Euglena gracilis the terminal four enzyme activities of the tryptophan biosynthetic pathway were found to be associated with a protein with an estimated molecular weight of 325,000 +/- 20,000. The protein was purified approximately 2,000-fold with relatively proportional recoveries of all four enzyme activities. The purified material was homogeneous by the criteria of analytical disc gel electrophoresis and gel isoelectric focusing. Disc gel electrophoresis after denaturation with sodium dodecyl sulfate gave a single protein band with a molecular weight of 155,000 +/- 5,000. Disc gel electrophoresis in 8 M urea also gave rise to a single protein band. We interpret these results as evidence for a single species of subunit. The pathway in Euglena is the only one known to the present in which the terminal enzyme, tryptophan synthase, is not a separate molecular species.