Ustilago maydis spermidine synthase is encoded by a chimeric gene, required for morphogenesis, and indispensable for survival in the host.

To analyze the role of spermidine in cell growth and differentiation of Ustilago maydis, the gene encoding spermidine synthase (Spe) was isolated using PCR. We found that the enzyme is encoded by a chimeric bifunctional gene (Spe-Sdh) that also encodes saccharopine dehydrogenase (Sdh), an enzyme involved in lysine biosynthesis. The gene contains a 5' region encoding Spe, followed, without a termination signal or a second initiation codon, by a 3' region encoding Sdh, and directs the synthesis of a single transcript that hybridizes with 3' or 5' regions' probes of the gene. The gene could not be disrupted in a wild-type strain, but only in a mutant defective in the gene encoding ornithine decarboxylase (Odc). Single spe-sdh mutants were isolated after sexual recombination in planta with a compatible wild-type strain. Mutants were auxotrophic for lysine and spermidine, but not for putrescine, and contained putrescine and spermidine, but not spermine. Putrescine in double mutants is probably synthesized from spermidine by the concerted action of polyamine acetyl transferase and polyamine oxidase. spe-sdh mutants were sensitive to stress, unable to carry out the yeast-to-mycelium dimorphic transition, and showed attenuated virulence to maize. These phenotypic alterations were reverted by complementation with the wild-type gene.

Investigations about N-aminopropyl transferases probably involved in biomineralization.

Polyamines are widespread distributed all over in living organisms. In Thalassiosira pseudonana 10 N-aminopropyl transferase like nucleotide sequences exists. It is assumed that these sequences are involved in the biomineralization of the diatom shell. The cDNA of the sequences were cloned, recombinant overexpressed and assayed with decarboxylated S-adenosylmethionine and several radioactive labelled polyamines. However, only a spermidine synthase and a thermospermine synthase were found to be enzymatically active in an in vitro assay. Both enzyme activities could be recognized in the crude extracts of Thalassiosira pseudonana and Cyclotella meneghiana. In further investigations the kinetics of the thermospermine synthase was determined and a site-specific mutagenesis of the bindig cavity of decarboxylated S-adenosylmethionine was carried out.