The genome of the pathogen Clostridium perfringens encodes two proteins, GerO and GerQ, homologous to monovalent cation transporters suggested to have roles in the germination of spores of some Bacillus species. GerO and GerQ were able to transport monovalent cations (K(+) and/or Na(+)) in Escherichia coli, and gerO and gerQ were expressed only in the mother cell compartment during C. perfringens sporulation. C. perfringens spores lacking GerO were defective in germination with a rich medium, KCl, L-asparagine, and a 1:1 chelate of Ca(2+) and dipicolinic acid (DPA), but not with dodecylamine, and the defect was prior to DPA release in germination. All defects in gerO spores were complemented by ectopic expression of wild-type gerO. Loss of GerQ had much smaller effects on spore germination, and these effects were most evident in spores also lacking GerO. A modeled structure of GerO was similar to that of the E. coli Na(+)/H(+) antiporter NhaA, and GerO, but not GerQ contained two adjacent Asp residues thought to be important in the function of this group of cation transporters. Replacement of these adjacent Asp residues in GerO with Asn reduced the protein's ability to complement the germination defect in gerO spores but not the ability to restore cation transport to E. coli cells defective in K(+) uptake. Together, these data suggest that monovalent cation transporters play some role in C. perfringens spore germination. However, it is not clear whether this role is directly in germination or perhaps in spore formation.