Characterization of the VOC Promoter That Is Active Under Low-Salinity Conditions in the Diatom Phaeodactylum tricornutum

Microalgae such as <i>Phaeodactylum tricornutum</i> are promising cell biofactories for the production of high-value molecules, including monoclonal antibodies (mAbs). However, to date, the production of mAbs in <i>P. tricornutum</i> using the inducible nitrate reductase (NR) promoter has yielded only a limited amount of mAbs. Therefore, the identification of a robust promoter that produces high yields of mAbs is crucial for the development of a cost-effective expression system. To date, only a few endogenous promoters have been characterized in <i>P. tricornutum</i>. In this study, we identified thirty-three potential "strong" endogenous promoters based on our previously published transcriptomic data from the <i>P. tricornutum</i> Pt3 strain. These putative promoter sequences were cloned into an episomal vector and fused to the gene encoding enhanced green fluorescent protein (eGFP). Their strength was assessed by measuring eGFP fluorescence, which reflects the level of eGFP protein expression. Of the thirty-three promoters, thirteen were able to successfully drive eGFP protein expression. Among them, the best results were obtained with the VOC promoter, which allowed a significant increase in eGFP expression compared to that induced by the NR promoter. These results contribute to the identification of new genetic tools that can be used in future studies to increase the yield of production of recombinant proteins in <i>P. tricornutum</i> at an industrial scale.