Transformation of the diatom Phaeodactylum tricornutum (Bacillariophyceae) with a variety of selectable marker and reporter genes

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Abstract
A general purpose transformation vector, designated pPha‐T1, was constructed for use with the diatom Phaeodactylum tricornutum Bohlin. This vector harbors the sh ble cassette for primary selection on medium containing the antibiotic zeocin, and a multiple cloning site flanked by the P. tricornutum fcpA promoter. pPha‐T1 was used to establish the utility of three selectable marker genes and two reporter genes for P. tricornutum transformation. The nat and sat‐1 genes confer resistance to the antibiotic nourseothricin, and nptII confers resistance to G418. Each of these genes was effective as a selectable marker for identifying primary transformants. These markers could also be used for dual selections in combination with the sh ble gene. The reporter genes uidA and gfp were also introduced into P. tricornutum using pPha‐T1. Gus expression in some transformants reached 15 μg·μg−1 of total soluble protein and permitted excellent cell staining, while GFP fluorescence was readily visible with standard fluorescence microscopy. The egfp gene, which has optimal codon usage for expression in human cells, was the only version of gfp that produced a strong fluorescent signal in P. tricornutum. The codon bias of the egfp gene is similar to that of P. tricornutum genes. This study suggests that codon usage has a significant effect on the efficient expression of reporter genes in P. tricornutum. The results presented here demonstrate that a variety of selectable markers and reporter genes can be expressed in P. tricornutum, enhancing the potential of this organism for exploring basic biological questions and industrial applications.