A novel salt-inducible CrGPDH3 promoter of the microalga Chlamydomonas reinhardtii for transgene overexpression

The expression of transgenes in the nucleus is an attractive alternative for the expression of recombinant proteins in the green microalga Chlamydomonas reinhardtii. For this purpose, a strong inducible promoter that allows protein accumulation without possible negative effects on cell multiplication and biomass accumulation is desirable. A previous study at our laboratory identified that the CrGPDH3 gene from C. reinhardtii was inducible under NaCl treatments. In this study, we cloned and characterized a 3012bp sequence upstream of the start codon of the CrGPDH3 gene, including the 285bp 5 untranslated region. This region was identified as the full-length promoter and named PromA (-2727 to +285). Deletion analysis of PromA using GUSPlus as a reporter gene enabled us to identify PromC (-653 to +285) as the core promoter, displaying basal expression. A region named RIA1 (-2727 to -1672) was suggested to contain the NaCl response elements. Moreover, deletion analysis of RIA1 enabled us to identify a region of 577bp named RIA3 (-2727 to -2150) that, when cloned upstream of PromC, was able to drive the expression of GUSPlus in response to 5 and 100mM NaCl, and 100mM KCl, similar to the native CrGPDH3 promoter. These results expand our understanding of the transcriptional mechanism of CrGPDH3 and clearly show that CrGPDH3 promoter and its chimeric forms are highly salt-inducible and can be used as inducible promoters for the overexpression of transgenes in C. reinhardtii.