Thymidylate synthase (TS) of Trichinella spiralis, a parasitic nematode causing trichinellosis, was found to bind its own mRNA and repress translation of the latter, similar to its human counter-part [Chu, Koeller, Casey, Drake, Chabner, Elwood, Zinn and Allegra (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 8977-8981]. However, in striking contrast with human TS, the parasite enzyme's interaction with mRNA was not affected by any of the substrate (deoxyuridylate or N(5,10)-methylenetetrahydrofolate) nor by the inhibitor (fluorodeoxyuridylate; used alone or in the presence of N(5,10)-methylenetetrahydrofolate) similar to that shown for the bifunctional enzyme from Plasmodium falciparum [Zhang and Rathod (2002) Science 296, 545-547]. Moreover, repression of the translation of the parasite enzyme was enhanced by the same ligands that were shown by others (Chu et al., 1991) to prevent human TS from impairing its translation. On comparing the capacity of TS to bind to its cognate mRNA, relative to its ability to inhibit its translation, the same enzyme preparation was active as translational repressor at a considerably lower protein/mRNA ratio, suggesting the two phenomena to be disconnected. Of interest is the fact that the presence of the enzyme protein N-terminal methionine proved to be critical for binding, but not for repression of its translation, indicating that mRNA binding requires a methionine or an adduct (i.e. methionine-histidine) at the N-terminus of TS, but that the translational repression effect does not. Notably, chicken liver dihydrofolate reductase, which is incapable of binding to T. spiralis TS mRNA, repressed the translation of TS.