Jan Pilny

Chromera velia is photosynthetic alga that is evolutionary closest to the group of het- erotrophic apicomplexan parasites. The main light harvesting carotenoid of C. velia has been described as isofucoxanthin-like pigment, however its structure remained unknown. Here, we purified this pigment from a 5 L culture of C. velia using a two-step HPLC procedure and its structure determined using combination of tandem mass spectrometry and 2D- and 3D-NMR techniques. The data identified this pigment as a new allenic xanthophyll which we denoted chromeraxanthin. Although previously predicted to be structurally similar to isofucoxanthin, chromeraxanthin possess different arrangement of the terminal cycles and thus it likely not derived directly from fucoxanthin biosynthe- sis pathway. The six membered ring connected to the carotenoid backbone via allenic bond is identical to that of fucoxanthin including its O-acetylation. However, the second six-membered ring bearing epoxy group in fucoxanthin is replaced by a capsanthin-like cyclopentane ring in chromeraxanthin. Notably, similar structure lacking only the O- acetylation has been previously described as allenicmytiloxanthin isolated from Pacific oyster.

Oxygenic photosynthesis relies on accessory factors to promote the assembly and maintenance of the photosynthetic apparatus in the thylakoid membranes. The highly conserved membrane-bound rubredoxin-like protein RubA has previously been implicated in the accumulation of both PSI and PSII, but its mode of action remains unclear. Here, we show that RubA in the cyanobacterium Synechocystis sp PCC 6803 is required for photoautotrophic growth in fluctuating light and acts early in PSII biogenesis by promoting the formation of the heterodimeric D1/D2 reaction center complex, the site of primary photochemistry. We find that RubA, like the accessory factor Ycf48, is a component of the initial D1 assembly module as well as larger PSII assembly intermediates and that the redox-responsive rubredoxin-like domain is located on the cytoplasmic surface of PSII complexes. Fusion of RubA to Ycf48 still permits normal PSII assembly, suggesting a spatiotemporal proximity of both proteins during their action. RubA is also important for the accumulation of PSI, but this is an indirect effect stemming from the downregulation of light-dependent chlorophyll biosynthesis induced by PSII deficiency. Overall, our data support the involvement of RubA in the redox control of PSII biogenesis.