An unprecedented combination of metabolic pathways in the cryptic plastid of a non-photosynthetic euglenophyte

Abstract

The non-photosyntetic alga Euglena longa harbours a cryptic plastid of unknown function. By a combination of bioinformatic and biochemical approaches we found out that this organelle houses a surprising set of metabolic processes. Biosynthesis of isoprenoid precursors and fatty acids is absent and the tetrapyrrole pathway is incomplete, whereas phospholipids and glycolipids are still being produced in the E. longa plastid. Unprecedented among non-photosynthetic plastids is the ability of this organelle to make tocopherols and a phylloquinone derivative. The most striking attribute is the presence of a linearized Calvin-Benson (CB) pathway including RuBisCO, together with ferredoxin-NADP⁺ reductase and the ferredoxin/thioredoxin system. We hypothesize that the linear CB pathway is regulated by the redox status of the E. longa cell, in effect functioning as a redox valve bypassing the glycolytic oxidation of glyceraldehyde-3-phosphate to 3-phosphoglycerate. Altogether, the E. longa plastid defines a new class of relic plastids.