A tenth atp gene and the conserved atpl gene of a Bacillus atp operon have a role in Mg²⁺ uptake

The atp operon of alkaliphilic Bacillus pseudofirmus OF4, as in most prokaryotes, contains the eight structural genes for the F-ATPase (ATP synthase), which are preceded by an atpl gene that encodes a membrane protein of unknown function. A tenth gene, atpZ, has been found in this operon, which is upstream of and overlapping with atpl. Most Bacillus species, and some other bacteria, possess atpZ homologues. AtpZ is predicted to be a membrane protein with a hairpin topology, and was detected by Western analyses. Deletion of atpZ, atpl, or atpZl from B. pseudofirmus OF4 led to a requirement for a greatly increased concentration of Mg²⁺ for growth at pH 7.5. Either atpZ, atpl, or atpZl complemented the similar phenotype of a triple mutant of Salmonella typhimurium (MM281), which is deficient in Mg²⁺ uptake. atpZ and atpl, separately and together, increased the Mg²⁺-sensitive ⁴⁵Ca²⁺ uptake by vesicles of an Escherichia coli mutant that is defective in Ca²⁺ and Na⁺ efflux. We hypothesize that AtpZ and Atpl, as homooligomers, and perhaps as heterooligomers, are Mg²⁺ transporter, Ca²⁺ transporter, or channel proteins. Such proteins could provide Mg²⁺, which is required by ATP synthase, and also support charge compensation, when the enzyme is functioning in the hydrolytic direction; e.g., during cytoplasmic pH regulation.