Arabidopsis polyamine biosynthesis: absence of ornithine decarboxylase andthe mechanism of arginine decarboxylase activity

Unlike other eukaryotes, which can synthesize polyamines only from ornithin e, plants possess an additional pathway from arginine. Occasionally non-enz ymatic decarboxylation of ornithine could be detected in Arabidopsis extrac ts; however, we could not detect ornithine decarboxylase (ODC; EC 4. 1.1.17 ) enzymatic activity or any activity inhibitory to the ODC assay. There are no intact or degraded ODC sequences in the Arabidopsis genome and no ODC e xpressed sequence tags. Arabidopsis is therefore the only plant and one of only two eukaryotic organisms (the other being the protozoan Trypanosome cr uzi) that have been demonstrated to lack ODC activity. As ODC is a key enzy me in polyamine biosynthesis, Arabidopsis is reliant on the additional argi nine decarboxylase (ADC; EC 4.1.1.9) pathway, found only in plants and some bacteria, to synthesize putrescine. By using site-directed mutants of the Arabidopsis ADC1 and heterologous expression in yeast, we show that ADC, li ke ODC, is a head-to-tail homodimer with two active sites acting in trans a cross the interface of the dimer. Amino acids K136 and C524 of Arabidopsis ADC1 are essential for activity and participate in separate active sites. M aximal activity of Arabidopsis ADC1 in yeast requires the presence of gener al protease genes, and it is likely that dimer formation precedes proteolyt ic processing of the ADC pre-protein monomer.