Fig. 5: Molecular basis of cytosine-dependent HJ resolution by NtMOC1.

a The C–G base pairs at the crossover are disrupted by NtMOC1. Left panel: Overall view of the disrupted C–G base pairs in the complex. Middle panel: The disrupted cytosine (C9) interacts with D183 via a hydrogen bond. Right panel: The flipped-out guanine is stabilized by residues and nucleotides. b, c HJ binding (b) and cleavage (c) activity examined for NtMOC1 with alanine substitutions of Y180 and D183. For the HJ binding and cleavage assay, X2 (CCGG) was used as HJ substrate. The final concentration of HJ in each lane was 250 nM. Five gradients with increasing concentrations (0, 125, 250, 500, and 1000 nM) were applied for each protein sample. The reactions were resolved by native PAGE and visualized by GelRed staining. Source data are provided as a Source Data file. d The BR loop undergoes a striking conformational change. Apo NtMOC1 is shown in the gray cartoon illustration. The protomers of HJ-bound MOC1 are shown in slate and yellow cartoons, respectively. DNA strands are shown with lines. The purple circle indicates the magnesium ion in the complex structure. The yellow dashed line indicates the hydrogen bond formed between D183 and cytosine. Arrows indicate the displacement of Y180 and D183. Residues in the apo and complex structures are labeled in black and magenta, respectively. The scissors indicate the cleavage site.