The DNA Polymerase III Holoenzyme: An Asymmetric Dimeric Replicative Complex with Leading and Lagging Strand Polymerases

The DNA Polymerase III holoenzyme forms initiation complexes on primed DNA in an ATP-dependent reaction. We demonstrate that the nonhydrolyzable ATP analog, ATPγS, supports the formation of an isolable leading strand complex that loads and replicates the lagging strand only in the presence of ATP, β, and the single-stranded DNA binding protein. The single endogenous DnaX complex within DNA polymerase III holoenzyme assembles β onto both the leading and lagging strand polymerases by an ordered mechanism. The dimeric replication complex disassembles in the opposite order from which it assembled. Upon ATPγS-induced dissociation, the leading strand polymerase is refractory to disassembly allowing cycling to occur exclusively on the lagging strand. These results establish holoenzyme as an intrinsic asymmetric dimer with distinguishable leading and lagging strand polymerases.