Metabolic and transcriptional response to the induction of a heterologous poly-3-hydroxybutyrate pathway in Phaeodactylum tricornutum

The marine diatom Phaeodactylum tricornutum is an emerging host for metabolic engineering, but little is known about how introduced pathways are integrated into the existing metabolic framework of the host and how this influences trans- and native-gene expression. In this study we used episomal vectors to express the heterologous poly-3-hydroxybutyrate (PHB) pathway in diatoms, which produces precursors for polymeric bioplastics from acetyl coenzyme-A (AcCoA). By experimentally perturbing cultivation conditions, we gained insight into the regulation of the endogenous metabolism in transgenic lines under various environmental scenarios. Biosynthesis of PHB led to distinct shifts in the metabolome of the host, and further analysis revealed a condition-dependent relationship between endogenous and transgenic metabolic pathways. Under N-limitation, which induced a significant increase in neutral lipid content, both metabolic and transcriptomic data suggest that AcCoA was preferentially shunted into the endogenous metabolism over the transgenic PHB pathway. In contrast, supply of organic carbon in the form of glycerol supported both fatty acid and PHB biosynthesis, suggesting cross-talk between cytosolic and plastidial AcCoA precursors. This is the first study to investigate the transcriptomic and metabolomic response of diatom cell lines expressing a heterologous multi-gene pathway under different environmental conditions, providing useful insights for future engineering attempts for pathways based on the precursor AcCoA. RNA-seq