Institut für Pharmazeutische Biologie, Technische Universität Braunschweig, Mendelssohnstrasse 1, 38106 Braunschweig, Germany.
Pyrrolizidine alkaloids (PAs) are preformed plant defense compounds with sporadic phylogenetic distribution. Homospermidine synthase (HSS) is the first specific enzyme in PA biosynthesis, and the link between primary and secondary metabolism. HSS was purified from roots of Senecio vernalis and microsequenced. Of the resulting peptides two did show homology to deoxyhypusine synthase (DHS). DHS catalyses the first step of the post-translational activation of the eukaryotic initiation factor 5A (eIF-5A). While HSS transfers the aminobutyl moiety of spermidine to putrescine, DHS transfers it in an analogous manner to the e -amino group of a specific lysine residue in the eIF-5A precursor protein, commencing the activation procedure of eIF-5A. DHS is already described from animals, fungi and archaebacteria, but not from plants. Using an alignment of all known DHS, degenerated primers were designed for RT-PCR, to identify, clone and express HSS and DHS from Senecio vernalis. The amino acid sequences of these two genes show 79% identity to each other and 61% identity to DHS described from human. Thus we are able to prove that the gene for HSS, involved in plant alkaloid biosynthesis, is derived from an ubiquitous, highly conserved gene involved in initiation of protein biosynthesis. Further experiments will show, whether the HSS in the various angiosperm lineages was recruited several times independently from DHS (polyphyletic origin).