Bifunctional thymidylate synthase-dihydrofolate reductase from Leishmania tropica: sequence homology with the corresponding monofunctional proteins.

In protozoa, thymidylate synthase (TS; EC 2.1.1.45) and dihydrofolate reductase (DHFR; EC 1.5.1.3) exist as a bifunctional protein. Full-length cDNA and genomic DNA clones encoding the bifunctional TS-DHFR from Leishmania tropica have been isolated, characterized, and sequenced. There are no intervening sequences within the coding region for TS-DHFR, and the G+C content is high, with a strong bias for codons possessing guanine or cytosine in the third base. From the predicted protein sequence for TS-DHFR, we conclude that the enzymes exist as separate structural domains with DHFR located at the amino terminus and TS at the carboxyl terminus; in between, there is a junctional peptide predicted to consist of a random coil flanked by two alpha-helices that corresponds to the site of a putative gene fusion. Comparison of the predicted amino acid sequence of L. tropica TS-DHFR with sequences of the monofunctional enzymes from other sources shows that the TS domain is much more conserved than is the DHFR domain and that both enzymes are more related to their monofunctional counterparts in vertebrates than in prokaryotes. Most of the recognized elements of secondary structure of other DHFRs, as ascertained by x-ray crystallography, are found in the predicted structure of the DHFR domain of the bifunctional protein.