Figure 7.

H1⁰ binding model. Step 1: Our cooperative binding analysis of H1⁰ constructs suggests that the majority of initial contact of H1⁰ (dark green) with a nucleosome (grey with red DNA) occurs through the C-terminal tail as the C mutant had a much higher affinity than the S1 or S2 mutants. Step 2: This restricts the search space of the globular domain, allowing either the S1 binding site (black oval) or the S2 binding site (white oval) to subsequently bind their target DNA (black square and white triangle, respectively) at rates that are cooperatively enhanced by γ ∼2.5 times compared with a freely diffusing molecule. Step 3: A conformational change occurs that brings the binding targets for S1 and S2 closer together so that they can be bound by a single globular domain at the same time (not possible in the S1S2 mutant because it lacks the C-terminal tail). This may coincide with the C-terminal tail acquiring structure (shown as a straighter tail) that strengthens the overall bond (the slow state present in the C, S1C, and S2C mutants), although our data cannot conclusively determine when this occurs. Step 4: This further restricts the search space of the globular domain, allowing the remaining binding site to subsequently bind at a rate that is cooperatively enhanced by γ∼25 times compared to a freely diffusing molecule.