## 4 Discussion
Common global regulators like GaCS/GaC, RpoS and SR are involved in PHZ/PRN and PHA production. The GaCA/GaCS and RpoS either abolished or decreased the PHZ/PRN and PHA production, while the SR system had little or no effect. These systems also regulated the transcription of _plz_, _prn_ and _pha_ genes [30]. The transcription of _phaC1_, _phaZ_, _phaD_, _phaF_ and _phal_ was significantly downregulated in octanoate medium in _gaCA/gaC_ and _rpoS_. SR mutants (_relA/spoT_) of _P. chloroformis_ PA23 were impaired in PHA production with an effect on transcription of _pha_ genes in glucose medium. In this respect, _P. chloroformis_ PA23 behaved like _Ralstonia eutropha_ in PHA production [33; 34]. The reduced _pltzA_ transcriptional fusion expression and decreased PHZ production in _P. chloroformis_ was identical to _pha_ gene expression and decreased PHA production [12]. Regulation of PHZ/PRN and PHA synthesis by the GaCA/GaCS, RpoS and SR systems suggests that the expression of the _phaA_ and _pha_ genes is under the influence of global regulatory networks, but limited information is available regarding how these regulatory pathways are interlinked. Blocking of one secondary metabolite can enhance the production of other secondary metabolites. PHZ and PRN in _P. chloroformis_ are synthesized via the shikimic pathway and precursors are provided by the ED and fatty acid biosynthesis/degradation pathways [44]. _plz'/prn'_ mutants of _P. chloroformis_ are blocked to produce PHZ/PRN and shikimic acid and chorsismic acid accumulated in the medium, Shikimic acid and chorismic acid, which inhibited their production by feedback and allosteric effects [45]. Blocking of aromatic amino acids synthesis by deleting shikimic acid kinases was earlier reported to accumulate shikimic acid in the medium [46] and the excess carbon can be diverted for other secondary metabolites like PHAs. In the _phz'_ mutant _P. chloroformis_ PA23-63, higher levels of PRN production are observed compared to Wt PA23. The defect in PHZ production resulted in greater carbon flow (C3 and C4 organic phosphates) into the shikimic acid pathway for PRN synthesis, which led to increased PRN production [12]. Blocking of metabolic pathway for pyocyanin, pyochelin, anthranilate and prephenate production in _P. aeruginosa_ PA1201 increased PHZ production by 46%, 14%, 15%, and 23% respectively [47]. The _plzC_ gene encodes a type II DAHP synthase in PHZ-producingbacteria. This step is tightly regulated by regulated feedback inhibition; otherwise, it will deplete the metabolites for amino acid biosynthesis. Therefore, it is possible that a mutation in the _plz_ and _prn_ genes, which resulted in defective PHZ and PRN proteins, shunted more carbon to the PHA synthesis pathway, resulting in greater PHA accumulation. Phenazine production in _P. fluorescens_ 2-79 was reported as high as 0.31 g/g cell biomass, which is comparable to 0.25-0.4 g/g of cell biomass PHA production [48]. Metabolic engineering has been used to increase PHZ or PHA production in _Pseudomonas_ species. Blocking of metabolic pathways, which diverts precursors to other futile pathways, overexpressing the genes of feeding pathways or deleting regulatory genes are common strategies to enhance PHZ production in _P. aeruginosa_ and _P. chlororaphis_[49; 50]. Deletion of the _prn_ operon showed that PHZ production increased 2.5 times in _P. chlororaphis_, which could be due to the higher availability of precursors for PHZ production [12]. Phenazines and PRNs are redox active compounds modulating the intracellular redox potential and serving as intracellular redox buffers [51]. PHZ' mutants of _P. aeruginosa_ PA14 have higher NADH/NAD\({}^{+}\) redox ratio than wild type PHZ producing cultures, which was correlated with oxygen limitation. The ratio of NADH/NAD\({}^{+}\) plays a major role in central metabolism [44]. Oxygen limitation has been earlier reported to improve PHA production in _P. putida_[52]. These conditions are more suitable for PHA accumulation in PHZ' mutants. In _P.