Differences in helix α1" geometry between in E. coli and S. pyogenes β. The dimeric interface of both β clamps distort of this helix (E. coli in blue, S. pyogenes in green). In the monomeric form of β (shown in yellow) crystallized bound to the δ subunit of the clamp loader , this helix is straight. The other protomer of S. pyogenes β is shown in grey for reference, with the sidechain of Phe81 shown in red. Figure rendered with with PyMol (DeLano Scientific).