Polyamines, putrescine, spermidine and spermine, are essential polycationic metabolites present in all eukaryotic cells, where they regulate fundamental processes including nucleic acid stabilization, translation, and stress responses. Spermidine synthase (SPDS), a member of the aminopropyltransferase (APT) family, catalyzes the transfer of an aminopropyl group from decarboxylated S-adenosylmethionine (dc-SAM) to putrescine to form spermidine. Although genomic analyses predict the presence of SPDS homologs in multiple fungal species, polyamine biosynthesis has not been experimentally characterized in the multidrug-resistant fungal pathogen Candidozyma auris. Here, we report the biochemical and functional characterization of the C. auris spermidine synthase, CauSpe3. The CauSPE3 gene complemented a Saccharomyces cerevisiae spe3Δ mutant demonstrating conserved function in vivo. Recombinant CauSpe3 was expressed in Escherichia coli, purified and analyzed using the fluorescence-based DAB-APT assay, which uses 1,2-diacetylbenzene (DAB) for polyamine detection. CauSpe3 catalyzed efficient conversion of putrescine to spermidine in the presence of dc-SAM, with Khalf values of 65.5 ± 7.11 µM for putrescine and 66.9 ± 2.09 µM for dc-SAM, and Vmax values of 7.1 ± 0.57 and 7.9 ± 0.12 nmol·µg−1·min−1, respectively. A catalytic-site mutant and heat-inactivated enzyme showed no detectable activity, and product formation was confirmed by means of thin-layer chromatography and mass spectrometry. These findings establish CauSpe3 as a functional spermidine synthase.
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