Sld2 is essential for the initiation of DNA replication, but the mechanism underlying its role in replication is not fully understood. The S-phase cyclin dependent kinase (S-CDK) triggers the association of Sld2 with Dpb11, and a phosphomimetic mutation of Sld2, Sld2T84D, functionally mimics the S-CDK phosphorylated state of Sld2. We report that Sld2T84D binds directly to the single-stranded (ss) DNA of two different origins of replication, and S-CDK phosphorylation of Sld2 stimulates the binding of Sld2 to origin ssDNA. Sld2T84D binds to a thymine-rich ssDNA region of the origin ARS1, and substitution of ARS1 thymines with adenines completely disrupts binding of Sld2T84D. Sld2T84D enhances the ability of origin ssDNA to pulldown Dpb11, and Sld2 binding to origin ssDNA may be important to allow Sld2 and Dpb11 to associate with origin DNA. We also report that Sld2T84D anneals ssDNA of an origin sequence. Dpb11 anneals ssDNA to low levels, and the addition of Sld2T84D with Dpb11 results in higher annealing activity than that of either protein alone. Sld2-stimulated annealing may be important for maintaining genome stability during the initiation of DNA replication.
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