Abstract
BackgroundSo far very limited knowledge exists on L-arginine catabolism in cyanobacteria, although six major L-arginine-degrading pathways have been described for prokaryotes. Thus, we have performed a bioinformatic analysis of possible L-arginine-degrading pathways in cyanobacteria. Further, we chose Synechocystis sp. PCC 6803 for a more detailed bioinformatic analysis and for validation of the bioinformatic predictions on L-arginine catabolism with a transcript analysis.ResultsWe have evaluated 24 cyanobacterial genomes of freshwater or marine strains for the presence of putative L-arginine-degrading enzymes. We identified an L-arginine decarboxylase pathway in all 24 strains. In addition, cyanobacteria have one or two further pathways representing either an arginase pathway or L-arginine deiminase pathway or an L-arginine oxidase/dehydrogenase pathway. An L-arginine amidinotransferase pathway as a major L-arginine-degrading pathway is not likely but can not be entirely excluded. A rather unusual finding was that the cyanobacterial L-arginine deiminases are substantially larger than the enzymes in non-photosynthetic bacteria and that they are membrane-bound. A more detailed bioinformatic analysis of Synechocystis sp. PCC 6803 revealed that three different L-arginine-degrading pathways may in principle be functional in this cyanobacterium. These are (i) an L-arginine decarboxylase pathway, (ii) an L-arginine deiminase pathway, and (iii) an L-arginine oxidase/dehydrogenase pathway. A transcript analysis of cells grown either with nitrate or L-arginine as sole N-source and with an illumination of 50 μmol photons m-2 s-1 showed that the transcripts for the first enzyme(s) of all three pathways were present, but that the transcript levels for the L-arginine deiminase and the L-arginine oxidase/dehydrogenase were substantially higher than that of the three isoenzymes of L-arginine decarboxylase.ConclusionThe evaluation of 24 cyanobacterial genomes revealed that five different L-arginine-degrading pathways are present in the investigated cyanobacterial species. In Synechocystis sp. PCC 6803 an L-arginine deiminase pathway and an L-arginine oxidase/dehydrogenase pathway represent the major pathways, while the L-arginine decarboxylase pathway most likely only functions in polyamine biosynthesis. The transcripts encoding the enzymes of the two major pathways were constitutively expressed with the exception of the transcript for the carbamate kinase, which was substantially up-regulated in cells grown with L-arginine.
Highlights
Very limited knowledge exists on L-arginine catabolism in cyanobacteria, six major L-argininedegrading pathways have been described for prokaryotes
L-arginine metabolism is more complex than the majority of other metabolic pathways in living organisms
This is due to (1) the occurrence of a biosynthetic branch point at the level of carbamoylphosphate, a precursor for Larginine and pyrimidine biosynthesis, (2) the fact that Larginine is a potential precursor of polyamines, (3) the fact that L-arginine can be a precursor of 4-aminobutyrate, having a role as neurotransmitter in mammals, (4) the function of L-arginine as a precursor for nitric oxide, acting as an abundant signal molecule in bacteria, mammals, and plants, and (5) the existence of an impressive variety of L-arginine-degrading pathways in eubacteria and archaea
Summary
Very limited knowledge exists on L-arginine catabolism in cyanobacteria, six major L-argininedegrading pathways have been described for prokaryotes. L-arginine metabolism is more complex than the majority of other metabolic pathways in living organisms This is due to (1) the occurrence of a biosynthetic branch point at the level of carbamoylphosphate, a precursor for Larginine and pyrimidine biosynthesis, (2) the fact that Larginine is a potential precursor of polyamines, (3) the fact that L-arginine can be a precursor of 4-aminobutyrate, having a role as neurotransmitter in mammals, (4) the function of L-arginine as a precursor for nitric oxide, acting as an abundant signal molecule in bacteria, mammals, and plants, and (5) the existence of an impressive variety of L-arginine-degrading pathways in eubacteria and archaea. Compared to heterotrophically-growing prokaryotes, L-arginine has specific additional roles in cyanobacteria, because some strains have an alternative carbon dioxide fixation pathway with carbamoylphosphate as the first carbon dioxide fixation product. This pathway leads to the formation of L-citrulline and subsequently to Larginine [1,2]. Cyanophycin has been shown to have a complex dynamic metabolism, which is not yet completely understood [612]
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