Establishment of a nitrate-reductase promoter-driven inducible expression system for metabolic engineering in the marine diatom Fistulifera solaris

Abstract

Microalgae have emerged as promising hosts for producing valuable compounds, including lipids and recombinant proteins. The oleaginous diatom Fistulifera solaris has shown potential for industrial applications. However, the constitutive expression of proteins often causes cellular toxicity, necessitating a controlled gene expression system to enhance productivity. In this study, we characterized the endogenous promoter of the nitrate reductase (NR) gene as an inducible expression system. The promoter was activated by nitrate and suppressed by ammonium. We also established a nitrate-spike method for controlled promoter activation by adding NaNO₃ at specific time points, which achieved strong induction without compromising the cellular lipid content. This study provides the first characterization of the NR promoter in an oleaginous diatom and demonstrates its use for controlled compound production. The inducible expression system developed in this study represents fundamental technology for enhancing the productivity of F. solaris.