Bioinformatic analysis of an unusual gene-enzyme relationship in the arginine biosynthetic pathway among marine gamma proteobacteria: implications concerning the formation of N-acetylated intermediates in prokaryotes
Authors: Xu Ying , Glansdorff Nicolas , Labedan Bernard 
Published: January 2006

Background

The N-acetylation of L-glutamate is regarded as a universal metabolic strategy to commit glutamate towards arginine biosynthesis. Until recently, this reaction was thought to be catalyzed by either of two enzymes: (i) the classical N-acetylglutamate synthase (NAGS, geneargA) first characterized inEscherichia coliandPseudomonas aeruginosaseveral decades ago and also present in vertebrates, or (ii) the bifunctional version of ornithine acetyltransferase (OAT, geneargJ) present in Bacteria, Archaea and many Eukaryotes. This paper focuses on a new and surprising aspect of glutamate acetylation. We recently showed that inMoritella abyssiandM. profunda, two marine gamma proteobacteria, the gene for the last enzyme in arginine biosynthesis (argH) is fused to a short sequence that corresponds to the C-terminal, N-acetyltransferase-encoding domain of NAGS and is able to complement anargAmutant ofE. coli. Very recently, other authors identified inMycobacterium tuberculosisan independent gene corresponding to this short C-terminal domain and coding for a new type of NAGS. We have investigated the two prokaryotic Domains for patterns of gene-enzyme relationships in the first committed step of arginine biosynthesis.

Results

TheargH-Afusion, designatedargH(A), and discovered inMoritellawas found to be present in (and confined to) marine gamma proteobacteria of theAlteromonas-andVibrio-like group. Most of them have a classical NAGS with the exception ofIdiomarina loihiensisandPseudoalteromonas haloplanktiswhich nevertheless can grow in the absence of arginine and therefore appear to rely on thearg(A)sequence for arginine biosynthesis. Screening prokaryotic genomes for virtualargH-X'fusions' whereXstands for a homologue ofarg(A), we retrieved a large number of Bacteria and several Archaea, all of them devoid of a classical NAGS. In the case ofThermus thermophilusandDeinococcus radiodurans, thearg(A)-like sequence clusters withargHin an operon-like fashion. In this group of sequences, we find the short novel NAGS of the type identified inM. tuberculosis. Among these organisms, at leastThermus,MycobacteriumandStreptomycesspecies appear to rely on this short NAGS version for arginine biosynthesis.

Conclusion

The gene-enzyme relationship for the first committed step of arginine biosynthesis should now be considered in a new perspective. In addition to bifunctional OAT, nature appears to implement at least three alternatives for the acetylation of glutamate. It is possible to propose evolutionary relationships between them starting from the same ancestral N-acetyltransferase domain. InM. tuberculosisand many other bacteria, this domain evolved as an independent enzyme, whereas it fused either with a carbamate kinase fold to give the classical NAGS (as inE. coli)or withargHas in marine gamma proteobacteria. Moreover, there is an urgent need to clarify the current nomenclature since the same gene nameargAhas been used to designate structurally different entities. Clarifying the confusion would help to prevent erroneous genomic annotation.