(A) Model of Streptococcus pyogenes β clamp surrounding DNA. (B) Electrostatic surface of S. pyogenes β subunit. Two views of the clamp are shown that differ by a ~180° rotation about the vertical axis. Red indicates regions of negative electrostatic potential, white indicates neutral regions, and blue indicates positive regions. Figure rendered with GRASP [26, 27] and PyMol (DeLano Scientific). The position on the face of the S. pyogenes clamp where the δ subunit of the clamp loader would be expected to bind is indicated based upon the co-crystal structure of the E. coli proteins .