Radioactive nucleotides were from Perkin Elmer and unlabeled nucleotides were from GE Healthcare. DNA modification enzymes were from New England Biolabs. DNA oligonucleotides were from Integrated DNA Technologies. Streptavidin was from Pierce/Thermo Scientific. 5 mg of streptavidin powder was resuspended in 0.5 ml distilled water to make a 10 mg/ml stock. D-Biotin (50 mM aqueous solution) was from Invitrogen/Molecular Probes. Protein concentrations were determined using the Bio-Rad Bradford Protein stain using BSA as a standard. E. coli SSB and S. cerevisiae CMG, Pol ε, RFC, PCNA and RPA were overexpressed and purified as previously described (Georgescu et al., 2014; Langston et al., 2014).