). Here, the LDC-encoding gene (ldc) of this bacterium was cloned and characterized. DNA sequencing analysis revealed that the amino acid sequence of S. ruminantium LDC is 35% identical to those of eukaryotic ornithine decarboxylases (ODCs; EC 4.1.1.17), including the mouse, Saccharomyces cerevisiae, Neurospora crassa, Trypanosoma brucei, and Caenorhabditis elegans enzymes. In addition, 26 amino acid residues, K69, D88, E94, D134, R154, K169, H197, D233, G235, G236, G237, F238, E274, G276, R277, Y278, K294, Y323, Y331, D332, C360, D361, D364, G387, Y389, and F397 (mouse ODC numbering), all of which are implicated in the formation of the pyridoxal phosphate-binding domain and the substrate-binding domain and in dimer stabilization with the eukaryotic ODCs, were also conserved in S. ruminantium LDC. Computer analysis of the putative secondary structure of S. ruminantium LDC showed that it is approximately 70% identical to that of mouse ODC. We identified five amino acid residues, A44, G45, V46, P54, and S322, within the LDC catalytic domain that confer decarboxylase activities toward both L-lysine and L-ornithine with a substrate specificity ratio of 0.83 (defined as the k cat /K m ratio obtained with L-ornithine relative to that obtained with L-lysine). We have succeeded in converting S. ruminantium LDC to form with a substrate specificity ratio of 58 (70 times that of wild-type LDC) by constructing a mutant protein, A44V/G45T/V46P/ P54D/S322A. In this study, we also showed that G350 is a crucial residue for stabilization of the dimer in S. ruminantium LDC.Ornithine decarboxylase (ODC) (EC 4.1.1.17) is an important enzyme for the biosynthesis of putrescine, a precursor of polyamines which are implicated in a wide variety of biological processes that include the synthesis of DNA, RNA, and protein in all living cells (26,30,31). Lysine decarboxylase (LDC) (EC 4.1.1.18), which exists in most bacteria, is involved in the biosynthesis of cadaverine, a molecule that participates in the closing of the porin channels in the outer membrane of Escherichia coli (5) and is also an essential component of the peptidoglycan of Selenomonas ruminantium, Veillonella alcalescens, V. parvula, and Anaerovibrio lipolytica, which are strictly anaerobic gram-negative bacteria (9,12,14,15). Previously, we reported that in these bacteria, cadaverine is transferred to the D-glutamic acid residue of a lipid intermediate for the synthesis of the cadaverine-containing peptidoglycan by cadaverine transferase (11,12,15,17). In S. ruminantium, cadaverine is constitutively synthesized from L-lysine (16) and its synthesis was completely prevented by DL-␣-difluoromethyllysine (DFML) and DL-␣-difluoromethylornithine (DFMO), which are irreversible inhibitors of LDC from Mycoplasma dispar (27) and eukaryotic ODC, respectively, resulting in growth inhibition due to the synthesis of the abortive peptidoglycan without cadaverine (11,15). These observations suggested that S. ruminantium ODC could decarboxylate L-lysine, as well as Lornithine. Accordingly,...