Repeated structure and possible gene duplications in high potential iron protein and rubredoxin.

The three-dimensional structures of bacterial high potential iron protein (HIPIP) and rubredoxin have been searched for repeats to test whether these molecules evolved by independent tandem gene duplications. HIPIP has no structural repeats in spite of the observed repeated pattern in the amino acid sequence from Rhodopseudomonas gelatinosa. Rubredoxin from Clostridium pasteurianum has repeated hairpin loops of ten alpha-carbon atoms on both sides of the active centre iron-sulphur complex, which can be superposed within a root mean square deviation of 0.84 A by rotating about a local pseudodyad axis. The structural repeat matches a weak repeat in the amino acid sequence. It is concluded that the sequence repeats in HIPIP are probably a coincidence but that rubredoxin may have evolved by gene duplication from a dimer of two primitive hairpin loops.