# Nutrient Sensing and Biofilm Modulation: The Example of L-arginine in _Pseudomonas_
## 3 L-arginine as a Pleiotropic Nutrient  
L-arginine is one of the most versatile nutrients leading to multiple effects on cell metabolism both in eukaryotes and prokaryotes. Beyond its role as a building block of proteins, this amino acid may fuel metabolism acting as both a carbon/nitrogen source [25] and an ATP source under specific environmental conditions [26]. Since L-arginine is at the crossroads of different metabolic pathways involving dedicated enzymes which in principle may compete for the same metabolite [27], it has been proposed that multiple intracellular L-arginine pools may accumulate, which are differentially accessible to various metabolic enzymes [28]. Interestingly, this differential metabolite accumulation has also been proposed and demonstrated for the second messenger c-di-GMP, whose regulation may involve differentiated local c-di-GMP pools within the cell [29]. We briefly present the metabolic and genetic organisation of L-arginine metabolism found in _Pseudomonas aeruginosa_ (Figure 1), which is the reference system for biofilm studies, considering that the complex metabolic network involving this amino acid could be different in diverse organisms [30]. Overall, the pathways seem to be conserved for the most part in other _Pseudomonas_, based on available in silico information ([[https://www.genome.jp/kegg/](https://www.genome.jp/kegg/)]([https://www.genome.jp/kegg/](https://www.genome.jp/kegg/)) (accessed on 23 March 2022)). L-arginine biosynthesis proceeds from L-glutamate via ornithine; drawing on the carbamoyl phosphate pool, ornithine is converted into L-arginine, and the expression of the enzymes catalyzing these final steps are sensitive to L-arginine depletion [31]. L-arginine catabolism, on the other hand, includes four different routes which touch different aspects of cellular metabolism. The main route is the arginine succinyltransferase pathway (AST), which sustains the succinate/L-glutamate pool and the ammonia pool (carbon, nitrogen and energy). This metabolism is relevant under aerobic conditions, and the corresponding enzymes are encoded by the _aru_ operon, which is L-arginineinducible together with the glutamate dehydrogenase isoform encoded by the \(gdhB\) gene. L-arginine degradation is controlled in _P. aeruginosa_ by two-component response regulator CbrA/CbrB [32], whose cascade induces the expression of transcriptional activator ArgR responsible for L-arginine biosynthesis repression and L-arginine catabolism activation [33]. The other two catabolic routes, less represented, are the L-arginine decarboxylase (ADC) and dehydrogenase (ADH) pathways, the latter re-evaluated as the aminotransferase pathway (ATA) [34; 31]. These are controlled by other metabolites, particularly polyamines, rather than L-arginine. A fourth peculiar pathway is the arginine deaminase (ADI) one, which is the most relevant for L-arginine utilization under anaerobic conditions, given that it allows _P. aeruginosa_ to sustain ATP production from carbamoyl phosphate in place of the oxygen-dependent respiratory chain [35]. The ADI pathway, encoded by the _arc_ operon, is mainly regulated  
Figure 1: **L-Arginine metabolism in _Pseudomonas aeruginosa_ in a metro-style map. Nodes represent the most relevant metabolite(s) while different colors represent different metabolic categories. Each route represents the metabolic pathway named in the Figure with the same color, linking different nodes. The color code used for the nodes: (i) magenta and light navy nodes for C and N source metabolites, respectively; (ii) green for energy production; (iii) rust orange for other N metabolites. 2 NHCO®: carbamoyl phosphate. **  
by the ANR transcription factor [36], ArgR being a helper to augment the transcription activation [37]; ANR is a global regulator of anaerobic metabolism, and is also responsible for the induction of the nitric oxide (NO)-sensitive DNR transcription factor [38].