Form X on a linear fragment with closed ends. Hairpin oligonucleotides were added by ligation to the ends of purified Form X. The ligation products were gel-purified and analyzed on a polyacrylamide gel with or without preincubation at 100°C or in 0.1N NaOH. Lanes 1 and 2: linear fragment with closed ends, unincubated (lane 1) or incubated at 100°C (lane 2). Lanes 3-5: Form X with closed ends, unincubated (lane 3), incubated at 100°C (lane 4), or incubated in 0.1N NaOH (lane 5). Lanes 6 and 7: Form X with one end closed and the other end open, unincubated (lane 6) or incubated at 100°C (lane 7). It is observed that Form X with both ends closed is completely resistant to denaturation (lanes 4 and 5).