## 3 Discussion  
Antibiotic biosynthesis has long been known to be triggered in condition of phosphate limitation that results in energetic stress and repressed in condition of phosphate proficiency. In consequence, polyphosphates (polyP) as ancient and ubiquitous phosphate polymers, specifically mobilized in condition of phosphate and energetic stress, might be involved in the regulation of antibiotic biosynthesis. We thus investigated the role of proteins encoded by genes of the _ppk-pptA-SLI_4382_ operon, annotated as possibly involved in polyP metabolism, in the regulation of antibiotic biosynthesis in _S. Lividans_. This study and previous studies revealed that the expression of the two-component system PhoR/PhoP and of genes under its positive control, involved in Pi supply and saving, was strongly up-regulated in the _pptA_ (Figure 6a,b) and _ppk_ mutants [17]. This indicated that these mutants suffered from phosphate stress/low phosphate availability. The phosin PptA possesses a CHAD domain forming a positively charged tunnel thought to accommodate negatively charged polyP [19; 20]. This polyP binding protein is thus most likely involved in the degradation of polyP to supply phosphate intracellularly. The simplest hypothesis would be that PptA acts as an exo-polyphosphatase (PPX-enzyme). However, no similarities nor conserved motifs could be found between PptA and any known exo-polyphosphatases and a purified protein related to SLI_4383/PptA originating from _S. chartreusis_[20] did not show any exo-polyphosphatase activity in vitro (data not shown). We thus propose that the function of PptA would be to maintain polyphosphates in a suitable conformation for their degradation into phosphate by PPX-like enzymes or by more general phosphatases. PptA would thus constitute an accessory factor for these enzymes. The generation of phosphate from polyphosphate would be totally impaired in the _pptA_ mutant but would remain at a low level in the _ppk_ mutant since _pptA_ is weakly expressed from its own promoter in this strain. The phosphate limitation would thus be more severe in the _pptA_ mutant than in the _ppk_ mutant but far less severe in these two mutant strains than in _S. coelicolor_. Consistently, the _pptA_ mutant produces ACT earlier and more abundantly than the _ppk_ mutant but less abundantly than _S. coelicolor_. The _pptA_ mutant had a lipid content lower than that of the _ppk_ mutant after 72 h but much higher than that of _S. coelicolor_. Interestingly, after 72 h, the total lipid content of the _ppk_ mutant increased whereas that of the _pptA_ mutant decreased. The reduced total lipid content of these mutant strains was most likely due to their lower phospholipid content due to low phosphate availability [26] as well as to the consumption of acetylcholine to support the activation of the oxidative metabolism of the strains to re-establish their energetic balance, as proposed previously for _S. coelicolor_[5; 40]. The increase of the total lipid/phospholipid content of the _ppk_ mutant after 72 h likely indicates enhanced phosphate availability, likely due to the expression of _pptA_ from its own promoter taking place in this strain but not in the _pptA_ deletion mutant. Concerning the enhanced antibiotic production of these strains, we have previously reported that the great antibiotic production of _S. coelicolor_ is, at least in part, linked to the weak expression in this strain, of the two-component system PhoR/PhoP that positively controls phosphate supply and saving [22]. In consequence, _S. coelicolor_ is severely limited in phosphate and suffers from energetic stress. We proposed that the sensing of such stress triggers homeostatic processes that contribute to the re-establishment of the energetic balance of the strain [5; 25]. One of these processes is a strong reduction of phospholipids in order to save phosphate [26].