Replacement of an Anabolic Threonine Deaminase by a Catabolic Threonine Deaminase during Development of Dictyostelium discoideum

Development of the cellular slime mold, Dictyostelium discoideum, is fueled in part by endogenous reserves of protein. Several enzymes of amino acid metabolism have previously been found to be synthesized during the early stages of development. We have now found that cells of D. discoideum contain two distinct threonine deaminase enzymes (EC 4.2.1.16); one is sensitive to feedback inhibition by isoleucine and is most likely a biosynthetic enzyme, while the other is insensitive to isoleucine and may function catabolically. The two enzymes also differ in thermosensitivity and stability to freezing and thawing. The isoleucine-sensitive enzyme is synthesized in growing cells but is inactivated shortly after the initiation of development. The isoleucine-insensitive enzyme accumulates during the aggregation and pseudoplasmodial stages of development. The accumulation requires concomitant protein synthesis and a prior period of RNA synthesis. The enzyme of differentiating cells is able to catalyze the breakdown of serine as well as threonine and thus may be involved in catabolism of both of these amino acids. The period of accumulation of threonine deaminase is modified in a pair of temporally deranged mutant strains which complete morphogenesis in two-thirds or three times the normal time, respectively. Thus it appears that differentiation threonine deaminase is a developmentally regulated enzyme of D. discoideum.