A high-yield protein expression platform in the unicellular red alga <i>Cyanidioschyzon merolae</i>

The production of engineered proteins in transgenic cells is widely used in research, medicine and industry. However, conventional cell-based production systems still face challenges in cost, scalability and biosafety. Here, we present a recombinant protein expression platform with simplified purification based on the photosynthetic unicellular red alga Cyanidioschyzon merolae, which can be cultivated under highly acidic conditions using only inorganic nutrients, air, water and light. We first identified a promoter that drives high-level constitutive gene expression throughout the cell cycle, resulting in substantial mRNA accumulation in C. merolae. A stable transformant expressing His-tagged mVenus under the control of this promoter accumulated the recombinant protein to more than 1% of total soluble protein. The simple cellular architecture of C. merolae, including the absence of a cell wall, enables efficient protein extraction via a single freeze-thaw cycle, followed by purification using immobilized metal affinity chromatography (IMAC), yielding ∼13.9 mg of functional recombinant protein per gram of total soluble protein. Owing to its low cost, scalability, operational simplicity and minimal risk of contamination, this Cyanidioschyzon-based platform offers a practical and promising approach to recombinant protein production in a photosynthetic eukaryote.