SUMMARY: Seventy two Tn5 transposon insertions were isolated in the operon carried on the multicopy plasmid pFRD79. The polar nature of these mutations permitted examination of the expression and localization of the polypeptides in novel subunit combinations. The minimal catalytic unit is the FRDA plus B dimer. A transposon within (::Tn5) produces inactive, soluble FRDA polypeptide which has covalently attached 8α(3-histidyl)flavin adenine dinucleotide cofactor. A transposon mutation within (::Tn5) produces soluble, catalytically active dimer. An insertion in (::Tn5) produces both a soluble trimer composed of FRDABC, and a tetramer of FRDABC and truncated FRDD bound to the inner membrane. Eighty percent of the activity is in the soluble form. Using this mutant, the requirement for FRDD both for optimal activity of the catalytic domain and for proper anchorage in the cytoplasmic membrane was demonstrated.


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