The Presence and Localization of Thioredoxins in Diatoms, Unicellular Algae of Secondary Endosymbiotic Origin

Abstract

Diatoms are unicellular algae of great ecological importance. So far, very little is known about the regulation of carbon fixation in these algae; however, there are strong indications that in diatom plastids, the ferredoxin/thioredoxin system might play a minor role in redox regulation of the photosynthetic reactions compared to land plants. Until now, it is unknown whether there are fewer or other target enzymes of thioredoxins in diatoms. Only a single potential target enzyme for thioredoxin, the plastidic fructose-1,6-bisphosphatase, has yet been identified. Nevertheless, during the annotation of the genome of the diatom Phaeodactylum tricornutum, we identified several genes encoding different thioredoxins. Utilizing in vivo expression of GFP:presequence fusion proteins in P. tricornutum, we were able to show that these thioredoxins are targeted either into plastids, mitochondria, or remain in the cytosol. Surprisingly, two of the three usually cytosolic thioredoxin h proteins are apparently plastid associated and, together with a thioredoxin reductase, putatively located in the periplastidic compartment. This is one of the few indications for so far unknown enzymatic reactions in the space between the two pairs of diatom plastid envelope membranes.

Evidence for glycoprotein transport into complex plastids

“…Besides such synthetic constructs, we screened all known endogenous PPC proteins (9,10,13,16,(29)(30)(31) for potential N-glycosylation sites using the program NetNGlyc 1.0. Interestingly, 23% of the 52 proteins possess potential N-glycosylation sites with high confidence predictions (++) within the mature, soluble protein (Table S1).…”

Carbon Fixation in Diatoms

“…Eight genes encoding a set of Trxs have been identifi ed in the genome of P. tricornutum , and at least three Trx candidates ( f , m , and y ) are located in the chloroplast together with a ferredoxin-thioredoxin reductase (FTR) gene, essential for Trx reduction via the electron transport chain (Weber et al 2009 ). However, none of the several key enzymes in the Calvin cycle, which are under redox regulation in higher plants, appear to be regulated by Trxs in diatoms (Liaud et al 2000 ;Michels et al 2005 ;Kroth et al 2008 ;Weber et al 2009 ), with the so far only exception of the fructose-1,6-bisphosphatase (FBPase) and the phosphoglycerate kinase (Michels et al 2005 ;Bosco et al 2012 ).…”

“…However, none of the several key enzymes in the Calvin cycle, which are under redox regulation in higher plants, appear to be regulated by Trxs in diatoms (Liaud et al 2000 ;Michels et al 2005 ;Kroth et al 2008 ;Weber et al 2009 ), with the so far only exception of the fructose-1,6-bisphosphatase (FBPase) and the phosphoglycerate kinase (Michels et al 2005 ;Bosco et al 2012 ). It is thus likely that the plastidic Trx systems in diatoms and in other chromist algae have undergone a unique functional specialization.…”

Regulation of glyceraldehyde-3-phosphate dehydrogenase in the eustigmatophytePseudocharaciopsis ovalisis intermediate between a chlorophyte and a diatom

“…However, many thioredoxins are targeted into the diatom plastid (Weber et al, 2009) and GAPDH in A. formosa is regulated by DTT (Boggetto et al, 2007;Erales et al, 2008;Maberly et al, 2010) indicating that it could be a possible target for thioredoxins. We found here that the activation of GAPDH by DTT was rapid in the three species (between 1.4 and 6 min), and fast enough to be ecophysiologically significant.…”