Fig. 4: The leading ssDNA inside the C-tier motor ring of CMG helicase.

a A side view of the DNA in the CMG chamber. The DNA-translocating PS1 and H2I loops of only four Mcm subunits (Mcm6, Mcm2, Mcm5, and Mcm3) spiral around and make contact with the leading ssDNA. b An illustration showing that among the four Mcm subunits engaging the DNA, each PS1 loop binds two phosphates along with an H2I loop binding over a base. The DNA-translocating loops rise two bases per Mcm subunit. c A bottom view showing the positively charged spiral path of the leading ssDNA formed by the PS1 and H2I loops of Mcm2, Mcm3, Mcm5, and Mcm6. d A top view of the PS1 and H2I loops in Mcm 3, 5, 2 and 6 engaging the spiral leading ssDNA. The middle and right panels show the DNA-interacting positively charged residues of H2I (middle) and PS1 (right). e Sequence alignment of the H2I and PS1 loops among the four DNA-interacting Mcm subunits. Invariant residues are highlighted in green, identical residues in yellow, and similar residues in cyan.