An FAD assembly protein, which accelerates covalent attachment of the cofactor into other proteins (PubMed:22474332, PubMed:26644464, PubMed:24374335). Plays an essential role in the assembly of succinate dehydrogenase (SDH, respiratory complex II), an enzyme complex that is a component of both the tricarboxylic acid cycle and the electron transport chain, and which couples the oxidation of succinate to fumarate with the reduction of ubiquinone (coenzyme Q) to ubiquinol. Required for flavinylation (covalent attachment of FAD) of the flavoprotein subunit SdhA of SDH (PubMed:24374335, PubMed:26644464, PubMed:22474332). Required for flavinylation of the flavoprotein subunit FdrA of fumarate reductase (FDR) (PubMed:26644464). Binds 2 different sites on the flavoprotein target FrdA (and presumably also SdhA), possibly positioning FAD and protein to facilitate the covalent bond formation; covalent attachment of FAD is not required for SDH or FDR complex enzyme formation (PubMed:26644464). Overexpression of this protein and YgfX (formerly cptA) restores production of prodigiosin antibiotic (Pig) in Serratia strains with deletions of sdhE-ygfX (PubMed:22474332). Monomer (PubMed:15593094). Can be cross-linked to FrdA and SdhA (PubMed:26644464). No aerobic growth on succinate, 80% reduction of succinate dehydrogenase (SDH) activity (PubMed:22474332). Significantly decreased anaerobic growth on glycerol fumarate medium, 70% reduction in fumarate reductase activity (PubMed:24374335). Low covalent attachment of FAD to FdrA or SdhA (PubMed:26644464). Belongs to the SdhE FAD assembly factor family. Was originally thought to be the antitoxin component of a type II toxin-antitoxin (TA) system (PubMed:22239607). When coexpressed with cognate toxin CptA (now ygfX), the antitoxin neutralizes toxicity of CptA (PubMed:22239607). Has been suggested to be a transcriptional regulator (PubMed:22239607). The putative toxin, CptA (ygfX) has since been shown not to be toxic in E.coli or Serratia species (PubMed:23657679).