The Eyes of Diatoms: Molecular Mechanisms of Aureochromes in the Diatom Phaeodactylum tricornutum

Abstract

Diatoms are unicellular microalgae thriving in various aquatic habitats, utilizing photoreceptors to precisely detect light quantity and quality (color) and adjusting their physiology accordingly. Aureochromes (AUREOs) are blue light receptors that also function as bZIP (basic leucine zipper) transcription factors. The model diatom Phaeodactylum tricornutum encodes four Aureo genes: PtAureo1a, 1b, 1c, and 2. PtAUREO1a has been shown to significantly impact overall gene regulation and light acclimation. However, the molecular mechanisms of PtAUREO1a's gene regulation remain unclear and only little is known about other PtAUREO paralogs. Here, we study the biological roles of individual PtAUREOs and unravel their functional mechanisms. We demonstrate the distinct light absorptions of individual PtAUREOs and investigate the disparate protein stabilities of PtAUREO1a and PtAUREO1b upon a shift from red light to blue light: PtAUREO1a is rapidly degraded by the proteasome, while PtAUREO1b shows no significant changes. These findings suggest distinctive functions of individual PtAUREOs with different light sensitivities and represent the first experimental demonstration of a functional proteasome-ubiquitin system in P. tricornutum. Using pull-down assays, we verify that all combinations of PtAUREO dimers can be formed, suggesting the cooperative functions of PtAUREOs and the potential expansion of their target genes. Using a modified yeast one-hybrid assay, we identify PtAUREO1a’s target genes, including transcription factors like PtbZIP10, PtAureo1a, PtAureo1c, PtbHLH2_PAS, and PtHSF3.2b. These genes may trigger further signal cascades, thus enabling PtAUREO1a to indirectly regulate a wide range of genes. We found that PtAUREO1a and PtAUREO1b can initiate gene transcription without blue light illumination and that PtAUREO1a may not require either blue light or an Aureo box, a known binding motif of AUREOs (5’-TGACGT-3’), for gene regulation. By examining mutant lines of P. tricornutum expressing a mutated PtAUREO1a with a significant loss of light absorption (PtBlindA1a), we find novel evidence that PtAUREO1a impacts PtLhcf15 gene expression, which is essential for red light acclimation. Also, through characterizing TALEN-mediated PtAureo1c knock-out mutants, we reveal that PtAUREO1c is involved in red light acclimation, photoprotection, and cell cycle regulation. In conclusion, PtAUREOs are transcription factors that finely tune gene expression and cell physiology in response to light signals, which may contribute to diatoms' high acclimation capacity in various environments.