Cation-dependent Reassociation of Subunits of N10-Formyltetrahydrofolate Synthetase from Clostridium acidi-urici and Clostridium cylindrosporum

N10-Formyltetrahydrofolate synthetase from Clostridium acidi-urici or Clostridium cylindrosporum is composed of four subunits which are identical by the criteria of molecular weight, peptide mapping, and isoelectric focusing in 8 m urea. The enzyme dissociates to inactive monomeric subunits in the absence of monovalent cations and reassociates to greater than 80% upon readdition of cations. The reassociation process involves cations in at least two steps. Steps I has little specificity and the cation requirement can be eliminated with ADP or ATP. The second step has an absolute requirement for monovalent cations where the order of selectivity is: NH4+ g K+, Rb+, Cs+ g Na+, Li+. At saturating concentrations of NH4Cl the rate of reassociation is first order with respect to protein which suggests that a conformational change of protein following binding of cation in Step II is the rate-determining process. No species intermediate between monomer and tetramer were observed.