Paramecium bursaria Chlorella Virus-1 Encodes an Unusual Arginine Decarboxylase That Is a Close Homolog of Eukaryotic Ornithine Decarboxylases

Abstract

Paramecium bursaria chlorella virus (PBCV-1) is a large double-stranded DNA virus that infects chlorellalike green algae. The virus encodes a homolog of eukaryotic ornithine decarboxylase (ODC) that was previously demonstrated to be capable of decarboxylating L-ornithine. However, the active site of this enzyme contains a key amino acid substitution (Glu for Asp) of a residue that interacts with the ␦-amino group of ornithine analogs in the x-ray structures of ODC. To determine whether this active-site change affects substrate specificity, kinetic analysis of the PBCV-1 decarboxylase (PBCV-1 DC) on three basic amino acids was undertaken. The k cat /K m for L-arginine is 550-fold higher than for either L-ornithine or L-lysine, which were decarboxylated with similar efficiency. In addition, ␣-difluoromethylarginine was a more potent inhibitor of the enzyme than ␣-difluoromethylornithine. Mass spectrometric analysis demonstrated that inactivation was consistent with the formation of a covalent adduct at Cys 347 . These data demonstrate that PBCV-1 DC should be reclassified as an arginine decarboxylase. The eukaryotic ODCs, as well as PBCV-1 DC, are only distantly related to the bacterial and plant arginine decarboxylases from their common ␤/␣-fold class; thus, the finding that PBCV-1 DC prefers L-arginine to L-ornithine was unexpected based on evolutionary analysis. Mutational analysis was carried out to determine whether the Asp-toGlu substitution at position 296 (position 332 in Trypanosoma brucei ODC) conferred the change in substrate specificity. This residue was found to be an important determinant of substrate binding for both Larginine and L-ornithine, but it is not sufficient to encode the change in substrate preference.

Evolution and Multifarious Horizontal Transfer of an Alternative Biosynthetic Pathway for the Alternative Polyamine sym-Homospermidine

“…P. bursaria Chlorella virus-1 (PCBV-1) has a 331-kbp genome) that infect single-celled eukaryotes such as the green alga Chlorella, an endosymbiont of the ciliate P. bursaria. The presence of hss in the chloroviral genomes is intriguing because these genomes also contain a biosynthetic pathway consisting of an unusual arginine decarboxylase (58), an agmatine deiminase and N-carbamoylputrescine amidohydrolase (59), that together produce putrescine from arginine. An HSS orthologue from PCBV-1 was shown previously to be functional, producing homospermidine from two molecules of putrescine (39).…”

Mouse Ornithine Decarboxylase-like Gene Encodes an Antizyme Inhibitor Devoid of Ornithine and Arginine Decarboxylating Activity

“…Moreover, although it was suggested that arginine decarboxylation reaction in the rodent brain may be catalyzed by ODC (49), later studies reported that rat ADC was able to decarboxylate both arginine and ornithine, this enzyme being distinct from ODC (44). In this regard, a viral gene, a close homologue of eukaryotic ODC, has been shown to code for an enzyme capable of decarboxylating L-arginine preferentially to L-ornithine (50). Recently, Regunathan and co-workers (51) have identified a human cDNA clone that exhibits ADC activity when expressed in COS-7 cells.…”

Paramecium bursaria Chlorella Virus 1 Encodes a Polyamine Acetyltransferase

“…However, the four polyamines detected in the PBCV-1 virion (putrescine, cadaverine, spermidine, and homospermidine) are unlikely to be important in neutralizing its DNA because the number of polyamine molecules per virion could neutralize only ϳ0.2% of the DNA phosphate residues (5). Presumably, polyamines play an essential role in the PBCV-1 replication cycle because PBCV-1 encodes four functional polyamine biosynthetic enzymes, arginine/ornithine decarboxylase, agmatine iminohydrolase, N-carbamoylputrescine amidohydrolase, and homospermidine synthase (HSS) (5,6,27). The first three enzymes can convert arginine to putrescine, which is an important intermediate in the synthesis of spermidine, spermine, and homospermidine (supplemental Fig.…”