Evaluating nuclear transgene expression systems in Chlamydomonas reinhardtii

Chlamydomonas offers several potential advantages as a single-celled autotrophic recombinant protein production system including: facile transformation systems for all three genomes, low production costs, and the ability to secrete proteins. However, transgene expression levels from the nuclear genome of Chlamydomonas are often inadequate for industrial applications. With the objective of optimizing nuclear transgene expression, we surveyed transgene (luciferase) expression driven by seven different nuclear gene promoters and three different transcription terminators. The results demonstrate that in addition to the choice of nuclear gene promoters used, transcriptional terminators can have strong influence on transgene expression. We show that the psaD terminator improved transgene expression when paired with a variety of different gene promoters. Among those tested, the psaD-psaD promoter-terminator expression cassette gave the highest expression levels. This expression cassette was then used to express a human protein of pharmaceutical value, human butyrylcholinesterase (huBuChE). Chlamydomonas cells were able to accumulate the luciferase-huBuChE fusion protein to 0.4% of total soluble protein levels, which is comparable to the expression levels of the same protein obtained in plants. In addition, this study found that the form (linear or supercoiled) of the transforming DNA used for Chlamydomonas transformation had significant impact on the transformation efficiency and the level of transgene expression and stability. These results demonstrate that transgene expression in Chlamydomonas can potentially be scaled for commercial production of recombinant proteins.