Photosystem I complexes associated with fucoxanthin-chlorophyll-binding proteins from a marine centric diatom, Chaetoceros gracilis

Abstract

Diatoms occupy a key position as a primary producer in the global aquatic ecosystem. We developed methods to isolate highly intact thylakoid membranes and the photosystem I (PS I) complex from a marine centric diatom, Chaetoceros gracilis. The PS I reaction center (RC) was purified as a super complex with light-harvesting fucoxanthin-chlorophyll (Chl)-binding proteins (FCP). The super complex contained 224 Chl a, 22 Chl c, and 55 fucoxanthin molecules per RC. The apparent molecular mass of the purified FCP-PS I super complex (approximately 1000 kDa) indicated that the super complex was composed of a monomer of the PS I RC complex and about 25 copies of FCP. The complex contained menaquinone-4 as the secondary electron acceptor A1 instead of phylloquinone. Time-resolved fluorescence emission spectra at 77 K indicated that fast (16 ps) energy transfer from a Chl a band at 685 nm on FCP to Chls on the PS I RC complex occurs. The ratio of fucoxanthin to Chl a on the PS I-bound FCP was lower than that of weakly bound FCP, suggesting that PS I-bound FCP specifically functions as the mediator of energy transfer between weakly bound FCPs and the PS I RC.

Diatom Phytochromes Reveal the Existence of Far-Red-Light-Based Sensing in the Ocean

“…Treatment with the photosystem II inhibitor DCMU prior to FR light exposure had no effect on the expression of these genes ( Figure 3D), which further rules out any regulation of their expression by feedback from photosynthesis. Any photosystem I-mediated regulation can also be excluded given the absence of absorption of photosystem I complexes above 710 nm (Veith and Büchel, 2007;Ikeda et al, 2008). On the contrary, the Class 2 genes are maximally induced by the shorter wavelengths and their expression profiles do not match the Pt-DPH spectra of any of the two forms ( Figure 3E).…”

Menaquinone as Well as Ubiquinone as a Crucial Component in the Escherichia coli Respiratory Chain

“…Green filamentous bacteria dominantly utilize MK-10 both for photosynthetic and respiratory electron transport reactions (Hale et al, 1983). MK-4 is present in the PSI of the cyanobacterium Gloeobacter violaceus or Synechococcus PCC 7002 diatoms and primitive red alga C. caldarum (Ikeda et al, 2008;Mimuro et al, 2005;Sakuragi et al, 2005). Green sulfur bacteria contain MK-7 in their PSI-type RCs (Hauska et al, 2001), similar to that of PSI of green filamentous bacteria.…”

Macro-organization and structural flexibility of the photosynthetic pigment system in diatoms

“…It has also been shown that PSI forms a large supercomplex together with FCP: a monomeric PSI RC binds about 25 FCPs (Ikeda et al 2008). These data hence provide important but far not complete information about the localization of FCP in diatoms.…”

“…PsaM is supposed to play a role in the trimerization of the PSI complex in cyanobacteria, however the sequence similarity is only 50% (Veith et al 2007). The polypeptide composition of the PSI core is also varying between the diatom species (Berkaloff et al 1990;Ikeda et al 2008). It has been shown that PSI of diatoms exists in monomeric form (Veith et al 2007), in contrast to cyanobacteria, where PSI is trimeric.…”

“…Beyond PsaA and PsaB, the PSI core of diatoms contains PsaE, PsaL and PsaM as specific PSI proteins Ikeda et al 2008). PsaM is supposed to play a role in the trimerization of the PSI complex in cyanobacteria, however the sequence similarity is only 50% (Veith et al 2007).…”