The enzyme threonine deaminase (TD) is a key regulatory enzyme in the pathway for the biosynthesis of isoleucine. TD is inhibited by its end product, isoleucine, and this effect is countered by valine, the product of a competing biosynthetic pathway. Sequence and structure analyses have revealed that the protomers of many TDs have C-terminal regulatory domains, composed of two ACT-like subdomains, which bind isoleucine and valine, while others have regulatory domains of approximately half the length, composed of only a single ACT-like domain. The regulatory responses of TDs from both long and short sequence varieties appear to have many similarities, but there are significant differences. We describe here the allosteric properties of Bacillus subtilis TD (bsTD), which belongs to the short variety of TD sequences. We also examine the effects of several mutations in the regulatory domain on the kinetics of the enzyme and its response to effectors. The behavior of bsTD can be analyzed and rationalized using a modified Monod-Wyman-Changeux model. This analysis suggests that isoleucine is a negative effector, and valine is a very weak positive effector, but that at high concentrations valine inhibits activity by competing with threonine for binding to the active site. The behavior of bsTD is contrasted with the allosteric behavior reported for TDs from Escherichia coli and Arabidopsis thaliana, TDs with two subdomains. We suggest a possible evolutionary pathway to the more complex regulatory effects of valine on the activity of TDs of the long sequence variety, e.g., E. coli TD.