These two organisms therefore use different mechanisms to extrude Ca2+ from the cell cytoplasm. These differences may be important since Ca2+ plays roles in development and antibiotic production in both organisms [72–75]. Sco also has two phosphate transporters of the Pit family although Mxa has only one. Both organisms have two or three members of the Na+:H+ Antiporter (NhaA) PS 341 Family.

Both also have multiple members of the functionally related Cation:Proton Antiporter (CPA1 and CPA2) Families (6 and 9 for Mxa and Sco, respectively). Both bacteria have five members of the CPA2 Family, but they have one and four members of the CPA1 family, respectively. Although members of these two families are within the same superfamily, they are only distantly related. The general reactions catalyzed by members of these families are similar, but most CPA1 family members transport Na+ while many CPA2 family members transport K+ (see TCDB). They are involved in pH and inorganic cation homeostasis [76]. A single multicomponent cation:H+ antiporter of the CPA3 Family is present in both organisms. Both organisms have a single ArsB arsenite exporter, but only Sco has two arsenite exporters of the Arsenical Resistance-3 (ACR3) Family. Mxa and Sco have 3 and 1 members of the DASS Family, respectively. Members of this family

take up both inorganic and organic anions, depending on the system. Both organisms have three paralogues of the SulP Family, which exclusively transport inorganic anions such as sulfate and bicarbonate. They also have two or three Silmitasertib molecular weight members of the Dicarboxylate/Amino Acid:Cation (Na+ or H+) Symporter (DAACS) and Bile Acid:Na+ Symporter (BASS) families which exclusively transport organic anions including amino acids. The two nucleobase:cation symporter families, NCS1 and NCS2, are prevalent in Sco (8 members), but appear to be lacking in Mxa. Both Sco and Mxa have TatA and TatC homologues, the essential constituents of the Sec-independent twin arginine translocase protein secretion system Carnitine palmitoyltransferase II [77]. However, while Sco has a 3-component system with TatA, B and C, Mxa appears to have a 2-component

system with just one TatA/B homologue [78]. Many prokaryotes have either 2 or 3 component systems, but the advantages of the greater complexity of the 3-component systems are not well understood, although distinct but overlapping functions for the E. coli TatA and TatB paralogues are recognized [77, 78]. The MOP Superfamily of multidrug/oligosaccharidyl lipid/polysaccharide exporters [79] is present in both organisms with Mxa having 7 members and Sco having 3. In Sco, one is probably a multidrug resistance pump while the other two may catalyze export of lipid-peptidoglycan precursors to the periplasm for cell wall assembly, as suggested by Ruiz [80]. B. subtilis has four such homologues, one of which, SpoVB, is required for spore cortex polymerization [81].