Polyamines are involved in many fundamental cellular processes. Common polyamines are putrescine, spermidine and spermine. Spermine is synthesized by transfer of an aminopropyl residue derived from decarboxylated S-adenosylmethionine to spermidine. Thermospermine is an isomer of spermine and assumed to be synthesized by an analogous mechanism. However, none of the recently described spermine synthases was investigated for their possible activity as thermospermine synthases. In this work, putative spermine synthases from the diatom Thalassiosira pseudonana and from Arabidopsis thaliana could be identified as thermospermine synthases. These findings may explain the previous result that two putative spermine synthase genes in Arabidopsis produce completely different phenotypes in knock-out experiments. Likely, part of putative spermine synthases identifiable by sequence comparisons represents in fact thermospermine synthases.

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a b s t r a c tPolyamines are ubiquitously present in all organisms. In addition to the common polyamines, thermophilic archaea synthesize long-chain polyamines. In the present study polyamine synthases from Hyperthermus butylicus and Pyrobaculum aerophilum were cloned and their substrate specificity was analyzed. The polyamine synthase HbSpeE II from H. butylicus synthesized long-chain polyamines with high activity using the same mechanism that is used by a wide range of organisms to synthesize common polyamines, in which the aminopropyl residue derives from decarboxylated S-adenosylmethionine. This is the first polyamine synthase described that synthesizes a polyamine longer than a tetramine with high activity.