Strand Displacement by DNA Polymerase Ill Occurs through a τ-ψ-χ Link to Single-stranded DNA-binding Protein Coating the Lagging Strand Template.

In addition to the well characterized processive replication reaction catalyzed by the DNA polymerase III holoenzyme on single-stranded DNA templates, the enzyme possesses an intrinsic strand displacement activity on flapped templates. The strand displacement activity is distinguished from the single- stranded DNA-templated reaction by a high dependence upon single-stranded DNA binding protein and an inability of γ-complex to support the reaction in the absence of τ. However, if γ-complex is present to load β2, a truncated r protein containing only domains Ill-V will suffice, This truncated protein is sufficient to bind both the a subunit of DNA polymerase (Pol) III and χψ. This is reminiscent of the minimal requirements for Pol III to replicate short single-stranded DNA-binding protein (SSB)-coated templates where τ is only required to serve as a scaffold to hold Pol III and χ in the same complex (Glover, B., and McHenry, C. (1998)1. Biol. Chem, 273, 23476 -23484). We propose a model in which strand displacement by DNA poly- merase III holoenzyme depends upon a Pol III-τ-ψ-χ-SSB binding network, where SSB is bound to the displaced strand, stabilizing the Pol Ill-template interaction. The same interaction network is probably important for stabilizing the leading strand polymerase interactions with authentic replication forks. The specificity constant (kcat/Km) for the strand displacement reaction is ∼300-fold less favorable than reactions on single-stranded templates and proceeds with a slower rate (150 nucleotides/s) and only moderate processivity (∼300 nucleotides). PriA, the initiator of replication restart on collapsed or misasasembled replication forks, blocks the strand displacement reaction, even if added to an ongoing reaction. [ABSTRACT FROM AUTHOR]