A superfamily of ATPases is described with its members sharing three distinct conserved amino acid sequence motifs. The superfamily includes numerous proteins involved in active partitioning of bacterial plasmids and chromosomes, nitrogenase iron proteins (nifH gene products), the anion pump ATPase ArsA, and VirC1 proteins encoded by Agrobacterium Ti plasmids and apparently involved in formation of single-stranded plasmid DNA. A database search identified partial sequences of genes encoding putative human and archaebacterial chromosome partitioning ATPases, suggesting that these proteins fulfil a universal function in cell division. The proteins belonging to this superfamily show the transition from the classical fingerprint of the A type purine NTP-binding motif, GXXGXGK[ST], to a significantly modified signature, KGGXXK[ST], with the apparent preservation of the loop conformation typical of this motif. It is speculated that the ancestral form of the A motif might have comprised a loop rich in Gly residues, GXGGXGK[ST], resembling that in NifH proteins and ArsA. Some of the Gly residues might have been differentially substituted in various evolutionary lineages of NTPases. The functional diversity of the proteins of this ATPase superfamily is comparable with the range of functions described previously for the superfamily of "UvrA-related" ATPases.