Figure 1.

OsO₄ footprint of the plasmid rENKpCRII756 containing (TG)₂₈, (TC)₂₀ repeats and nonrepetitive sequences showing calcium-induced structural changes indicated by reactive thymidines. (A) Supercoiled top (lanes 2–9) and bottom (lanes 11–13) strands and linearized top strands (lanes 14–16) were probed with OsO₄ without calcium or with increasing calcium concentrations in the absence or presence of 100 mM KCl. G reaction ladders from Maxam–Gilbert sequencing of the corresponding sequences were loaded as standards (lanes 1 and 10). Solid arrowhead, 5′ reactive site at position −618; star on lane 9, Hoogsteen structure formed in the middle of (TC)₂₀ sequences at high divalent cation concentrations. (B) The positions of OsO₄ reactive thymidines in rENKpCRII756 (arrows) indicating the calcium-induced structural change, with increasing size indicating increased reactivity. The arrow pointing to the most reactive thymidine at position −618 is circled. Two reactive thymidines at the (TG)₂₈/(TC)₂₀ junction are indicated with solid circles. The star marks the central reactive thymidine of the Hoogsteen structure in the middle of (TC)₂₀; T1, T15, and T28 mark the positions of the various reactive thymidines within the (TG)₂₈ repeat. Horizontal bars mark the boundaries of the two regions within the repeat with distinct OsO₄ reactivity.