In diatoms, the process of energy-dependent chlorophyll fluorescence quenching (qE) has an important role in photoprotection. Three components are essential for qE: (1) the light-dependent generation of a transthylakoidal proton gradient; (2) the deepoxidation of the xanthophyll diadinoxanthin (Dd) into diatoxanthin (Dt); and (3) specific nucleus-encoded antenna proteins, called Light Harvesting Complex Protein X (LHCX). We used the model diatom Phaeodactylum tricornutum to investigate the concerted light acclimation response of the qE key components LHCX, proton gradient, and xanthophyll cycle pigments (Dd+Dt) and to identify the intracellular light-responsive trigger. At high-light exposure, the up-regulation of three of the LHCX genes and the de novo synthesis of Dd+Dt led to a pronounced rise of qE. By inhibiting either the conversion of Dd to Dt or the translation of LHCX genes, qE amplification was abolished and the diatom cells suffered from stronger photoinhibition. Artificial modification of the redox state of the plastoquinone (PQ) pool via 3-(3,4-dichlorophenyl)-1,1-dimethylurea and 5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone resulted in a disturbance of Dd+Dt synthesis in an opposite way. Moreover, we could increase the transcription of two of the four LHCX genes under low-light conditions by reducing the PQ pool using 5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone. Altogether, our results underline the central role of the redox state of the PQ pool in the light acclimation of diatoms. Additionally, they emphasize strong evidence for the existence of a plastid-to-nucleus retrograde signaling mechanism in an organism with plastids that derived from secondary endosymbiosis.

In the yeast cell cycle ‘start’ requires sets of the Cdc10/ SWI family of transcriptional factors which activate the MCB cis elements contained in genes essential for S phase progression. Fission yeast possess two such overlapping systems, Res1‐Cdc10 and Res2‐Cdc10, both of which act to start the mitotic and meiotic cycles. We have recently isolated rep2+ as a multicopy suppressor of a temperature‐sensitive cdc10 mutant which encodes a zinc finger protein. Here we show that the Rep2 zinc finger protein is an essential component of the active Res2‐Cdc10 transcriptional regulator complex and likely to play a role in the control of cell cycle ‘start’. Our data suggest that Rep2 is a transcriptional activator subunit which interacts with the MCB binding subunit complex formed by Res2 and Cdc10.