Iron-sulfur cluster biosynthesis in algae with complex plastids

Abstract

Plastids surrounded by four membranes harbor a special compartment between the outer and inner plastidmembrane pair, the socalled periplastidal compartment (PPC). This cellular structure is usually presumed to be the reduced cytoplasm of a eukaryotic phototrophic endosymbiont, which was integrated into a host cell and streamlined into a plastid with a complex membrane structure. Up to date, no mitochondrion or mitochondrion-related organelle has been identified in the PPC of any representative. However, two prominent groups, the cryptophytes and the chlorarachniophytes, still harbor a reduced cell nucleus of symbiont origin, the nucleomorph, in their PPCs. Generally, many cytoplasmic and nucleus-located eukaryotic proteins need an iron-sulfur cofactor for their functionality. Beside someexceptions, their synthesis isdependingona so-callediron-sulfur complex (ISC) assembly machinery located in the mitochondrion. This machinery provides the cytoplasm with a still unknown sulfur component, which is then converted into iron-sulfur clusters via a cytosolic iron-sulfur protein assembly (CIA) machinery. Here, we investigated if a CIA machinery is present in mitochondrion-lacking PPCs. By using bioinformatic screens and in vivo-localizations of candidate proteins, we show that the presence of a PPC-specificCIAmachinery correlateswith the presence of a nucleomorph. Phylogenetic analyses of PPC- And host specificCIAcomponents additionally indicate a complex evolution of theCIAmachineries in organisms having plastids surrounded by four membranes.