ABSTRACT
Pseudomonas putida KT2440 is a model environmental bacterium attracting considerable interest as a cell factory through synthetic and systems biology approaches. Its metabolism has been extensively characterized, and high-quality metabolic models are available at the genomic scale, allowing for a holistic evaluation of its metabolic capabilities. Among its biotechnological applications, strain KT2440 is widely used for medium-chain-length PHA (mcl-PHA) production. Mcl-PHA metabolism is mediated through the catalytic action of the enzymes PHA polymerase (PhaC) and PHA depolymerase (PhaZ), which synthesize and degrade, respectively, PHA in a continuous cycle. In this strain, the PHA metabolism goes beyond solely having a role in carbon storage, since recent findings have highlighted its function as a key cycle that controls carbon and energy storage in the cells. The pha gene cluster is well conserved among the mcl-PHA production strains and contains two operons, phaC1ZC2D and phaFI, which are transcribed in opposite directions. PhaD transcriptionally activates the pha genes in a carbon source-dependent manner. PhaI and PhaF encode the phasins that have modular character. Besides structural functions, PhaF is responsible for granule segregation during cell division and is also involved in the expression of the pha genes, and to some extent, the entire transcriptomic profile under PHA production conditions. This effect is derived from its DNA-binding abilities and putative histone-like functionality. The expression of the genes in the pha cluster is also subject to the influence of global regulatory systems that are driven, inter alia, by the catabolite repression control (Crc) protein, rpoS sigma factor, the two-component GacS/GacA global regulatory system, and PsrA that negatively regulates the expression of genes related to fatty acid metabolism. In this book chapter, the molecular basis of PHA metabolism in P. putida will be described in depth.
