Bacterial initiators form dynamic filaments on single-stranded DNA monomer by monomer.

DNA replication initiation is mediated across all domains of life by initiator proteins oligomerizing at replication origins. Recently, it was shown that initiators can directly bind single-stranded DNA (ssDNA) and thus might enhance origin melting. In this study, we used single-molecule fluorescence assays to probe the ssDNA binding mechanism of the replication initiator DnaA. Our experiments revealed that DnaA forms a dynamic filament on ssDNA in 3' to 5' directionality in the presence of ATP and analogs. After nucleation with a three-monomer seed, monomers dynamically assemble and disassemble one monomer at a time at the 5' end, each monomer binding three nucleotides of ssDNA. The addition of adjacent double-stranded DnaA binding sites stabilized the DnaA filament on ssDNA. Our results extend the current models of origin melting via DnaA ssDNA interaction.