A strategy to complement PtAUREO1a in TALEN knockout strains of Phaeodactylum tricornutum

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The recent availability of genome editing tools like TALEN (Transcription activator-like effector nuclease) and CRISPR/Cas9 (Clustered regularly interspaced short palindromic repeats) for the diatom Phaeodactylum tricornutum has dramatically increased the options to explore diatom biology via reverse genetics. In order to verify that an observed phenotype indeed is directly related to a specific gene knockout and not due to a secondary effect, complementation of the inactivated gene with the wildtype gene and restoration of the wild type phenotype is an essential tool in molecular biology. So far, no strategy for a complementation method has been published for P. tricornutum. Here we demonstrate, as a proof-of principle, the complementation of P. tricornutum AUREO1a knockout strains previously created by TALEN technology. These strains are deficient in the PtAureo1a gene, which is encoding a blue-light dependent transcription factor. pPTbsr, a modified pPha-T1 vector with an antibiotic resistance cassette against Blasticidin served as a complementation vector. In order to avoid the modification of the complementing gene via the potentially still active TALEN nucleases, we have modified the TALEN binding sites of the complementing PtAureo1a gene using synonymous codons. The altered PtAureo1a gene along with its native promoter and terminator was transformed by particle gun bombardment into PtAUREO1a TALEN knockout strains of P. tricornutum. The integration and the expression of PtAUREO1a was confirmed by PCR and western blotting. Due to random integration within the genome, the expression level of the complemented gene may be variable in different lines. Physiological parameters indicated the successful rescue of the wild type phenotype in several lines that showed a similar PtAUREO1a protein content as wild type cells. Our method provides a rapid and efficient tool to complement knockout lines generated by genome editing approaches in P. tricornutum.