UNLABELLED: An analysis of the distribution of the Na(+)-translocating ATPases/ATP synthases among microbial genomes identified an atypical form of the F(1)F(o)-type ATPase that is present in the archaea Methanosarcina barkeri and M. acetivorans, in a number of phylogenetically diverse marine and halotolerant bacteria and in pathogens Burkholderia spp. In complete genomes, representatives of this form (referred to here as N-ATPase) are always present as second copies, in addition to the typical proton-translocating ATP synthases. The N-ATPase is encoded by a highly conserved atpDCQRBEFAG operon and its subunits cluster separately from the equivalent subunits of the typical F-type ATPases. N-ATPase c subunits carry a full set of sodium-binding residues, indicating that most of these enzymes are Na(+)-translocating ATPases that likely confer on their hosts the ability to extrude Na(+) ions. Other distinctive properties of the N-ATPase operons include the absence of the delta subunit from its cytoplasmic sector and the presence of two additional membrane subunits, AtpQ (formerly gene 1) and AtpR (formerly gene X). We argue that N-ATPases are an early-diverging branch of membrane ATPases that, similarly to the eukaryotic V-type ATPases, do not synthesize ATP. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.