Figure 1.

Structure and binding modes of EcSSB (A) Structural model of 65 nucleotides of ssDNA (red ribbon), wrapped around the EcSSB tetramer (8) in the (SSB)₆₅ mode. (B) Domain organization of EcSSB, depicting the DNA binding domain (OB), the C-terminal IDL and the 9-residue acidic ‘tip.’ (C) The proposed ssDNA binding pathways of the (SSB)₆₅ and (SSB)₃₅ modes of EcSSB (8,12). The EcSSB tetramer is depicted in blue with the ssDNA in red. ssDNA that passes along the backside of the schematic is depicted as a dotted line. OB-fold binding sites are represented with either an open circle, for an unoccupied binding site; a half-closed circled, for a partially occupied binding site; or a closed circle, for a fully occupied binding site (see Discussion for details of this model). Two EcSSB tetramers are shown in the (SSB)₃₅ mode to denote high cooperativity. The C-terminal tails are not depicted for clarity.