HypD is involved in hydrogenase formation. It contains many possible metal binding residues, which may bind to nickel. Transposon Tn5 insertions into hypD resulted in R. leguminosarum mutants that lacked any hydrogenase activity in symbiosis with peas [1].

1: Molecular analysis of a microaerobically induced operon required for hydrogenase synthesis in Rhizobium leguminosarum biovar viciae. Rey L, Murillo J, Hernando Y, Hidalgo E, Cabrera E, Imperial J, Ruiz-Argueso T; Mol Microbiol 1993;8:471-481. PMID:8326860