Purification of Fluorescent E. coli SSB

Both wild-type and fluorescently labeled E. coli SSB were expressed and purified as described previously (Lohman et al., 1986 (link); Roy et al., 2009 (link)), with an addition of a dsDNA cellulose column to remove a minor exonuclease contaminant (Bujalowski and Lohman, 1991b (link)). The labeled SSB was single-point mutated from Ala to Cys at position 122 in the C-terminus, and labeled with AlexaFluor555 maleimide (Invitrogen, Grand Island, NY) to the extent of ∼25% (∼1 dye per tetramer) as described previously (Roy et al., 2009 (link)). E. coli RecA was purchased from New England Biolabs (M0249S; Ipswich, MA).