Urea is the most utilized nitrogen fertilizer worldwide. For nitrogen availability urea needs to be hydrolyzed to ammonia and carbon dioxide. This reaction is catalyzed by the enzyme urease in plants and many microorganisms. Urease contains two nickel ions in each active center which are bound inter alia by a carbamylated lysine moiety. In plants the activation of urease requires three accessory proteins (urease accessory proteins: UAPs; UreD, UreF; UreG). We are interested in the molecular mechanisms underlying UAP function and investigate how these proteins generate the specificity for nickel in the urease activation process.