Plasmids are natural vectors for gene transfer. In Gram-negative bacteria, plasmid DNA replication is triggered when monomers of an initiator protein (Rep) bind to direct repeats at the origin sequence. Rep dimers, which are inactive as initiators, bind to an inverse repeat operator, repressing transcription of the rep gene. Rep proteins are composed of N-terminal dimerization and C-terminal DNA-binding domains. Activation of Rep is coupled to dimer dissociation, converting the dimerization domain into a second origin-binding module. Although the structure of the monomeric F plasmid initiator (mRepE) has been determined, the molecular nature of Rep activation remains unknown. Here we report the crystal structure of the dimeric N-terminal domain of the pPS10 plasmid initiator (dRepA). dRepA has a winged-helix fold, as does its homologous domain in mRepE. However, dimerization transforms an interdomain loop and beta-strand (monomeric RepE) into an alpha-helix (dimeric RepA). dRepA resemble the C terminus of eukaryotic and archaeal Cdc6, giving clues to the phylogeny of DNA replication initiators.