Additionally, it regenerates the NAD pool and keeps oxidative and reducing balance [30, 31]. Peroxiredoxin could act as protection factor against ROS generated by the stress caused by low polyP levels. Finally, increased levels of the translational factors EF-Tu and EF-Ts were found during polyP scarcity. This response has also been described in E. coli during acid stress and heavy metal (cobalt) exposure. It is suggested that these elongation factors could fold proteins in a way similar to that of stress chaperones [32]. Finally, as the GTP hydrolysis step is catalysed by EF-Tu, which binds to the large ribosomal subunit, it
has been proposed that the interaction between polyphosphate and the large ribosomal subunit promotes translation fidelity by influencing PARP inhibitor the EF-Tu GTPase reaction [33]. Altogether, these results suggest that during polyP scarcity a general stress state occurred and cells succeeded by overexpressing protein-folding chaperones. Transport proteins From the 17 total proteins identified whose expressions decreased during lack of polyP, 10 were identified as transporters. PS 341 Energy consuming ABC-type transporters responsible for carrying different solutes such
as sugars, peptides, polyamines, amino acids and Fe3+ were identified. Also, C4-dicarboxylates TRAP transporters and outer membrane protein OprE, which has been involved in virulence process in the genus Pseudomonas [34], were reduced in polyP(-) cells. Other processes and hypothetical proteins The present study also yielded some results that appear to be conflicting. We, and others, have demonstrated that despite the lack of motility of polyP-deficient Ribonucleotide reductase cells, the flagellum was intact (as seen by using transmission
electron microscopy). Nevertheless, we found flagellin, the major component of flagella filaments, diminished in the total and extracelullar proteome of polyP-deficient cells. Finally two protein spots present in the total proteome matched ORF sequences designated ‘hypothetical’ or ‘conserved hypothetical”" proteins. These hypothetical proteins identified here should be subjected to further characterization to confirm their possible role in polyP metabolism and to ascertain their true biological function. Discussion and Conclusions PolyP has numerous and diverse biological functions that have been discovered mainly by studying ppk1 mutants in bacteria. A P. aeruginosa PAO1 ppk1 null mutant exhibits pleiotropic phenotypes including decreased virulence, defective in motility, quorum sensing, biofilm formation and failure in responses to various stresses [13, 15, 22]. Many of these features were also observed in ppk1 mutants of other bacteria such as Vibrio cholerae, Salmonella, Shigella and others [35, 36].