Figure 5.

Electron microscopic analyses of S-DNAs +/– SSB. (A–D) Electron microscopic analyses of gel-purified (CTG)₅₀·(CAG)₅₀ DNAs; linear form (L), slipped homoduplex S-DNAs (S) and the (CTG)₅₀·(CAG)₃₀ and (CTG)₃₀·(CAG)₅₀ slipped intermediate heteroduplexes (SI). The repeat tracts are contained on an NdeI–HindIII fragment which has 265 and 63 bp 5′ and 3′, respectively, of the (CTG)_n tract. The left-most panels show schematic representations of the DNA structures, the middle panels show micrographs of the gel-purified naked DNA samples, and the right-most panels show micrographs of the same DNAs in the presence of single-strand binding protein (+ SSB). See text for details. Micrographs are shown in reverse contrast. Size bar in the lower right is 122 nm. (E) Summary of single-strand binding analyses; the qualitative and quantitative analyses of SSB binding were determined and tabulated. (F and G) Electron microscopic analyses of gel-purified sister SI-DNAs (CTG)₅₀·(CAG)₁₇ and (CTG)₁₇·(CAG)₅₀. The repeats are contained on the same NdeI–HindIII fragment as used for (A–D). The magnification for all panels is the same. The left-most panels show schematic representations of the DNA structures. The upper images of (F) and (G) show branched molecules with defined three-way slip-out junction with the slip-out in the hairpin conformation. The lower panels show ‘branchless’ molecules observed in the same gel-purified SI-DNA sample. Both a ‘branchless’ and a ‘branched’ molecule are seen in the lower micrograph of (F). The ‘branchless’ molecules had lengths similar to that of the n = 17 linear form (not shown), which electrophoretically migrated considerably faster than the SI-DNAs. The relative proportion of branched and branchless species in a given sample is indicated.