Electron transport accessory protein <p> The electron transport accessory proteins adopt the beta topology of an SH3 domain, with a partly opened beta barrel and a 3-10 helical turn interrupting the last strand. Proteins displaying this topology include R67 dihydrofolate reductase, which catalyses the hydration of nitriles to amides [<cite idref="PUB00006378"/>], photosystem I accessory proteins (PsaE), which participate in cyclic electron transport and modulate the interaction of photosystem I with ferrodoxin [<cite idref="PUB00010711"/>], the alpha chain of ferrodoxin thioredoxin reductase (FTR), which is involved in the regulation of the activity of chloroplast enzymes [<cite idref="PUB00010712"/>], and the beta chain of nitrile hydratase, which confers trimethoprim-resistance to R67-expressing bacteria [<cite idref="PUB00010713"/>]. </p>