Transcriptional Knockdown of GGPPS Increases Cellular FPP Availability and Heterologous Sesquiterpene Production in the Green Microalga Chlamydomonas reinhardtii

This study describes a genome-editing-based approach for transcriptional silencing of geranylgeranyl pyrophosphate synthase (GGPPS) expression in Chlamydomonas reinhardtii to investigate cellular farnesyl pyrophosphate (FPP) availability and to redirect isoprenoid precursors toward sesquiterpene biosynthesis. Using Cas9-mediated integration of a selection marker into the GGPPS promoter region, the assembly of the transcriptional complex was successfully disturbed. Two independent editing events resulted in stable knockdown mutants with position-dependent reductions in GGPPS expression, and the stronger knockdown caused decreases in total chlorophyll and carotenoid contents and increased isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) precursor pools in the chloroplast without impairing cell viability. Overexpression of various sesquiterpene synthases was used to assess the impact on heterologous terpenoid bioproduction, and specific bisabolene and patchoulol production were increased by more than 2-fold. Expression of the C. nootkatensis valencene synthase in combination with a heterologous farnesyl pyrophosphate synthase resulted in a 10.9-fold increase in specific valencene production. These results demonstrate that transcriptional interference via targeted DNA integration enables robust, position-dependent tuning of essential gene expression.