The cDNA encoding ornithine decarboxylase (ODC ; EC 4.1.1.17), a key enzyme in putrescine and polyamine biosynthesis, has been cloned from Nicotiana glutinosa (GenBank2 AF 323910), and was expressed in Escherichia coli. The amino acid sequence of N. glutinosa ODC showed 90 % identity with Datura stramonium ODC, and 44 % identity with human ODC. N. glutinosa ODC did not possess the PEST sequence [a sequence rich in proline (P), glutamic acid (E), serine (S) and threonine (T) residues] found in mammalian ODCs, which are thought to be involved in rapid degradation of the protein. The purified ODC was a homodimeric protein, having a native M r of 92 000. Kinetic studies of ODC showed that N. glutinosa ODC decarboxylated both -ornithine and -lysine with K m values of 562 µM and 1592 µM at different optimal pH values of 8.0 and 6.8 respectively. ODC activity was completely and irreversibly inhibited by α-difluoromethylornithine (K i 1.15 µM), showing a competitive inhibition pattern. Site-directed mutagenesis was performed on ODC to introduce mutations at conserved lysine Abbreviations used : ADC, arginine decarboxylase ; C377A, mutant bearing the site-directed mutation of Cys 377 Ala, etc. ; DFMO, α-
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