Figure 23-4: Model for SPP1 initiation of DNA replication. A, Model for SPP1 initiation of theta type DNA replication. First, G38P recognizes AB boxes of the SPP1 replication origin (oriL), and binds to them in an ATP-independent fashion. Binding of G38P leads to melting of the AT-rich region adjacent to the AB boxes. The helix destabilizing, G36P, binds to the ssDNA. The G39P-G40P-ATP complex is loaded in the unwound region by the interactions between G39P and G38P and the ATP-dependent ssDNA binding capacity of G40P. The interaction of G39P with G38P, which form a G38P-G39P heterodimer, dissociates G39P from the G39P-G40P-ATP complex and the helicase is free of the inhibitory effect of its loader with a subsequent release of the G39P monomers and homodimers and G38P-G39P heterodimers from the nucleoprotein complex. DnaG and both DNA pol III upon interacting with G40P are then loaded at the origin. The hydrolysis of ATP would then produce the unwinding of the DNA by G40P and its translocation. B, Roadblock as a model for the shift from theta to sigma replication. G38P bound to oriL or oriR blocks replication fork progression. A nick in the leading strand (bottom strand) will be processed by the putative 5-3 exonuclease, G34.1P, to generate a 3-ssDNA tail on which G35P will polymerize. A nick in the lagging strand (top strand), will not require further processing, and G35P will polymerise on it. C, Model for SPP1 initiation of sigma type DNA replication. Binding of G38P, with the help of G36P stabilizes the melted AT-rich region adjacent to the AB boxes at either oriL or oriR. A G35P-ssDNA filament pairs with the complementary strand of the unwound region to form a D-loop. The G39P-G40P-ATP complex is then loaded in the unwound region by the interactions between G39P and G38P and the ATP-dependent ssDNA binding capacity of G40P. The interaction of G39P with G38P, leading to G39P monomers and homodimers and G38P-G39P heterodimers, dissociates G39P from the G39P-G40P-ATP complex. G40P helps the assembly of DnaG and both DNA polIII at the AT-rich region. The 3-OH end of the paired strand could be used to prime the leading strand and DnaG could provide the primer for lagging strand synthesis. (for better image quality, click here for tif image, which may be quite large; may also require screen refreshing plus special software to view; or view larger tif in separate window)

Figure 23-1, Figure 23-2, Figure 23-3, Figure 23-4, Figure 23-5, Figure 23-6, Figure 23-7, Figure 23-8

return to Alonso, Tavares, Lurz, and Trautner, Bacteriophage SPP1